Nestled in a dense forest, the Ukrainian military base appears abandoned and destroyed, its command center a burned-out husk, a casualty of a Russian missile barrage early in the war.

But that is above ground.

Not far away, a discreet passageway descends to a subterranean bunker where teams of Ukrainian soldiers track Russian spy satellites and eavesdrop on conversations between Russian commanders. On one screen, a red line followed the route of an explosive drone threading through Russian air defenses from a point in central Ukraine to a target in the Russian city of Rostov.

The underground bunker, built to replace the destroyed command center in the months after Russia’s invasion, is a secret nerve center of Ukraine’s military.

There is also one more secret: The base is almost fully financed, and partly equipped, by the C.I.A.

“One hundred and ten percent,” Gen. Serhii Dvoretskiy, a top intelligence commander, said in an interview at the base.

Now entering the third year of a war that has claimed hundreds of thousands of lives, the intelligence partnership between Washington and Kyiv is a linchpin of Ukraine’s ability to defend itself. The C.I.A. and other American intelligence agencies provide intelligence for targeted missile strikes, track Russian troop movements and help support spy networks.

But the partnership is no wartime creation, nor is Ukraine the only beneficiary.

It took root a decade ago, coming together in fits and starts under three very different U.S. presidents, pushed forward by key individuals who often took daring risks. It has transformed Ukraine, whose intelligence agencies were long seen as thoroughly compromised by Russia, into one of Washington’s most important intelligence partners against the Kremlin today.

The listening post in the Ukrainian forest is part of a C.I.A.-supported network of spy bases constructed in the past eight years that includes 12 secret locations along the Russian border. Before the war, the Ukrainians proved themselves to the Americans by collecting intercepts that helped prove Russia’s involvement in the 2014 downing of a commercial jetliner, Malaysia Airlines Flight 17. The Ukrainians also helped the Americans go after the Russian operatives who meddled in the 2016 U.S. presidential election.

Around 2016, the C.I.A. began training an elite Ukrainian commando force — known as Unit 2245 — which captured Russian drones and communications gear so that C.I.A. technicians could reverse-engineer them and crack Moscow’s encryption systems. (One officer in the unit was Kyrylo Budanov, now the general leading Ukraine’s military intelligence.)

And the C.I.A. also helped train a new generation of Ukrainian spies who operated inside Russia, across Europe, and in Cuba and other places where the Russians have a large presence.

The relationship is so ingrained that C.I.A. officers remained at a remote location in western Ukraine when the Biden administration evacuated U.S. personnel in the weeks before Russia invaded in February 2022. During the invasion, the officers relayed critical intelligence, including where Russia was planning strikes and which weapons systems they would use.

“Without them, there would have been no way for us to resist the Russians, or to beat them,” said Ivan Bakanov, who was then head of Ukraine’s domestic intelligence agency, the S.B.U.

The details of this intelligence partnership, many of which are being disclosed by The New York Times for the first time, have been a closely guarded secret for a decade.

In more than 200 interviews, current and former officials in Ukraine, the United States and Europe described a partnership that nearly foundered from mutual distrust before it steadily expanded, turning Ukraine into an intelligence-gathering hub that intercepted more Russian communications than the C.I.A. station in Kyiv could initially handle. Many of the officials spoke on condition of anonymity to discuss intelligence and matters of sensitive diplomacy.

Now these intelligence networks are more important than ever, as Russia is on the offensive and Ukraine is more dependent on sabotage and long-range missile strikes that require spies far behind enemy lines. And they are increasingly at risk: If Republicans in Congress end military funding to Kyiv, the C.I.A. may have to scale back.

To try to reassure Ukrainian leaders, William J. Burns, the C.I.A. director, made a secret visit to Ukraine last Thursday, his 10th visit since the invasion.

From the outset, a shared adversary — President Vladimir V. Putin of Russia — brought the C.I.A. and its Ukrainian partners together. Obsessed with “losing” Ukraine to the West, Mr. Putin had regularly interfered in Ukraine’s political system, handpicking leaders he believed would keep Ukraine within Russia’s orbit, yet each time it backfired, driving protesters into the streets.

Mr. Putin has long blamed Western intelligence agencies for manipulating Kyiv and sowing anti-Russia sentiment in Ukraine.

Toward the end of 2021, according to a senior European official, Mr. Putin was weighing whether to launch his full-scale invasion when he met with the head of one of Russia’s main spy services, who told him that the C.I.A., together with Britain’s MI6, were controlling Ukraine and turning it into a beachhead for operations against Moscow.

But the Times investigation found that Mr. Putin and his advisers misread a critical dynamic. The C.I.A. didn’t push its way into Ukraine. U.S. officials were often reluctant to fully engage, fearing that Ukrainian officials could not be trusted, and worrying about provoking the Kremlin.

Yet a tight circle of Ukrainian intelligence officials assiduously courted the C.I.A. and gradually made themselves vital to the Americans. In 2015, Gen. Valeriy Kondratiuk, then Ukraine’s head of military intelligence, arrived at a meeting with the C.I.A.’s deputy station chief and without warning handed over a stack of top-secret files.

That initial tranche contained secrets about the Russian Navy’s Northern Fleet, including detailed information about the latest Russian nuclear submarine designs. Before long, teams of C.I.A. officers were regularly leaving his office with backpacks full of documents.

“We understood that we needed to create the conditions of trust,” General Kondratiuk said.

As the partnership deepened after 2016, the Ukrainians became impatient with what they considered Washington’s undue caution, and began staging assassinations and other lethal operations, which violated the terms the White House thought the Ukrainians had agreed to. Infuriated, officials in Washington threatened to cut off support, but they never did.

“The relationships only got stronger and stronger because both sides saw value in it, and the U.S. Embassy in Kyiv — our station there, the operation out of Ukraine — became the best source of information, signals and everything else, on Russia,” said a former senior American official. “We couldn’t get enough of it.”

This is the untold story of how it all happened.

A Cautious Beginning

The C.I.A.’s partnership in Ukraine can be traced back to two phone calls on the night of Feb. 24, 2014, eight years to the day before Russia’s full-scale invasion.

Millions of Ukrainians had just overrun the country’s pro-Kremlin government and the president, Viktor Yanukovych, and his spy chiefs had fled to Russia. In the tumult, a fragile pro-Western government quickly took power.

The government’s new spy chief, Valentyn Nalyvaichenko, arrived at the headquarters of the domestic intelligence agency and found a pile of smoldering documents in the courtyard. Inside, many of the computers had been wiped or were infected with Russian malware.

“It was empty. No lights. No leadership. Nobody was there,” Mr. Nalyvaichenko said in an interview.

He went to an office and called the C.I.A. station chief and the local head of MI6. It was near midnight but he summoned them to the building, asked for help in rebuilding the agency from the ground up, and proposed a three-way partnership. “That’s how it all started,” Mr. Nalyvaichenko said.

The situation quickly became more dangerous. Mr. Putin seized Crimea. His agents fomented separatist rebellions that would become a war in the country’s east. Ukraine was on war footing, and Mr. Nalyvaichenko appealed to the C.I.A. for overhead imagery and other intelligence to help defend its territory.

With violence escalating, an unmarked U.S. government plane touched down at an airport in Kyiv carrying John O. Brennan, then the director of the C.I.A. He told Mr. Nalyvaichenko that the C.I.A. was interested in developing a relationship but only at a pace the agency was comfortable with, according to U.S. and Ukrainian officials.

To the C.I.A., the unknown question was how long Mr. Nalyvaichenko and the pro-Western government would be around. The C.I.A. had been burned before in Ukraine.

Following the breakup of the Soviet Union in 1991, Ukraine gained independence and then veered between competing political forces: those that wanted to remain close to Moscow and those that wanted to align with the West. During a previous stint as spy chief, Mr. Nalyvaichenko started a similar partnership with the C.I.A., which dissolved when the country swung back toward Russia.

Now Mr. Brennan explained that to unlock C.I.A. assistance the Ukrainians had to prove that they could provide intelligence of value to the Americans. They also needed to purge Russian spies; the domestic spy agency, the S.B.U., was riddled with them. (Case in point: The Russians quickly learned about Mr. Brennan’s supposedly secret visit. The Kremlin’s propaganda outlets published a photoshopped image of the C.I.A. director wearing a clown wig and makeup.)

Mr. Brennan returned to Washington, where advisers to President Barack Obama were deeply concerned about provoking Moscow. The White House crafted secret rules that infuriated the Ukrainians and that some inside the C.I.A. thought of as handcuffs. The rules barred intelligence agencies from providing any support to Ukraine that could be “reasonably expected” to have lethal consequences.

The result was a delicate balancing act. The C.I.A. was supposed to strengthen Ukraine’s intelligence agencies without provoking the Russians. The red lines were never precisely clear, which created a persistent tension in the partnership.

In Kyiv, Mr. Nalyvaichenko picked a longtime aide, General Kondratiuk, to serve as head of counterintelligence, and they created a new paramilitary unit that was deployed behind enemy lines to conduct operations and gather intelligence that the C.I.A. or MI6 would not provide to them.

Known as the Fifth Directorate, this unit would be filled with officers born after Ukraine gained independence.

“They had no connection with Russia,” General Kondratiuk said. “They didn’t even know what the Soviet Union was.”

That summer, Malaysia Airlines Flight 17, flying from Amsterdam to Kuala Lumpur, blew up in midair and crashed in eastern Ukraine, killing nearly 300 passengers and crew. The Fifth Directorate produced telephone intercepts and other intelligence within hours of the crash that quickly placed responsibility on Russian-backed separatists.

The C.I.A. was impressed, and made its first meaningful commitment by providing secure communications gear and specialized training to members of the Fifth Directorate and two other elite units.

“The Ukrainians wanted fish and we, for policy reasons, couldn’t deliver that fish,” said a former U.S. official, referring to intelligence that could help them battle the Russians. “But we were happy to teach them how to fish and deliver fly-fishing equipment.”

A Secret Santa

In the summer of 2015, Ukraine’s president, Petro Poroshenko, shook up the domestic service and installed an ally to replace Mr. Nalyvaichenko, the C.I.A.’s trusted partner. But the change created an opportunity elsewhere.

In the reshuffle, General Kondratiuk was appointed as the head of the country’s military intelligence agency, known as the HUR, where years earlier he had started his career. It would be an early example of how personal ties, more than policy shifts, would deepen the C.I.A.’s involvement in Ukraine.

Unlike the domestic agency, the HUR had the authority to collect intelligence outside the country, including in Russia. But the Americans had seen little value in cultivating the agency because it wasn’t producing any intelligence of value on the Russians — and because it was seen as a bastion of Russian sympathizers.

Trying to build trust, General Kondratiuk arranged a meeting with his American counterpart at the Defense Intelligence Agency and handed over a stack of secret Russian documents. But senior D.I.A. officials were suspicious and discouraged building closer ties.

The general needed to find a more willing partner.

Months earlier, while still with the domestic agency, General Kondratiuk visited the C.I.A. headquarters in Langley, Va. In those meetings, he met a C.I.A. officer with a jolly demeanor and a bushy beard who had been tapped to become the next station chief in Kyiv.

After a long day of meetings, the C.I.A. took General Kondratiuk to a Washington Capitals hockey match, where he and the incoming station chief sat in a luxury box and loudly booed Alex Ovechkin, the team’s star player from Russia.

The station chief had not yet arrived when General Kondratiuk handed over to the C.I.A. the secret documents about the Russian Navy. “There’s more where this came from,” he promised, and the documents were sent off to analysts in Langley.

The analysts concluded the documents were authentic, and after the station chief arrived in Kyiv, the C.I.A. became General Kondratiuk’s primary partner.

General Kondratiuk knew he needed the C.I.A. to strengthen his own agency. The C.I.A. thought the general might be able to help Langley, too. It struggled to recruit spies inside Russia because its case officers were under heavy surveillance.

“For a Russian, allowing oneself to be recruited by an American is to commit the absolute, ultimate in treachery and treason,” General Kondratiuk said. “But for a Russian to be recruited by a Ukrainian, it’s just friends talking over a beer.”

The new station chief began regularly visiting General Kondratiuk, whose office was decorated with an aquarium where yellow and blue fish — the national colors of Ukraine — swam circles around a model of a sunken Russian submarine. The two men became close, which drove the relationship between the two agencies, and the Ukrainians gave the new station chief an affectionate nickname: Santa Claus.

In January 2016, General Kondratiuk flew to Washington for meetings at Scattergood, an estate on the C.I.A. campus in Virginia where the agency often fetes visiting dignitaries. The agency agreed to help the HUR modernize, and to improve its ability to intercept Russian military communications. In exchange, General Kondratiuk agreed to share all of the raw intelligence with the Americans.

Now the partnership was real.

Operation Goldfish

Today, the narrow road leading to the secret base is framed by minefields, seeded as a line of defense in the weeks after Russia’s invasion. The Russian missiles that hit the base had seemingly shut it down, but just weeks later the Ukrainians returned.

With money and equipment provided by the C.I.A., crews under General Dvoretskiy’s command began to rebuild, but underground. To avoid detection, they only worked at night and when Russian spy satellites were not overhead. Workers also parked their cars a distance away from the construction site.

In the bunker, General Dvoretskiy pointed to communications equipment and large computer servers, some of which were financed by the C.I.A. He said his teams were using the base to hack into the Russian military’s secure communications networks.

“This is the thing that breaks into satellites and decodes secret conversations,” General Dvoretskiy told a Times journalist on a tour, adding that they were hacking into spy satellites from China and Belarus, too.

Another officer placed two recently produced maps on a table, as evidence of how Ukraine is tracking Russian activity around the world.

The first showed the overhead routes of Russian spy satellites traveling over central Ukraine. The second showed how Russian spy satellites are passing over strategic military installations — including a nuclear weapons facility — in the eastern and central United States.

The C.I.A. began sending equipment in 2016, after the pivotal meeting at Scattergood, General Dvoretskiy said, providing encrypted radios and devices for intercepting secret enemy communications.

Beyond the base, the C.I.A. also oversaw a training program, carried out in two European cities, to teach Ukrainian intelligence officers how to convincingly assume fake personas and steal secrets in Russia and other countries that are adept at rooting out spies. The program was called Operation Goldfish, which derived from a joke about a Russian-speaking goldfish who offers two Estonians wishes in exchange for its freedom.

The punchline was that one of the Estonians bashed the fish’s head with a rock, explaining that anything speaking Russian could not be trusted.

The Operation Goldfish officers were soon deployed to 12 newly-built, forward operating bases constructed along the Russian border. From each base, General Kondratiuk said, the Ukrainian officers ran networks of agents who gathered intelligence inside Russia.

C.I.A. officers installed equipment at the bases to help gather intelligence and also identified some of the most skilled Ukrainian graduates of the Operation Goldfish program, working with them to approach potential Russian sources. These graduates then trained sleeper agents on Ukrainian territory meant to launch guerrilla operations in case of occupation.

It can often take years for the C.I.A. to develop enough trust in a foreign agency to begin conducting joint operations. With the Ukrainians it had taken less than six months. The new partnership started producing so much raw intelligence about Russia that it had to be shipped to Langley for processing.

But the C.I.A. did have red lines. It wouldn’t help the Ukrainians conduct offensive lethal operations.

“We made a distinction between intelligence collection operations and things that go boom,” a former senior U.S. official said.

‘This is Our Country’

It was a distinction that grated on the Ukrainians.

First, General Kondratiuk was annoyed when the Americans refused to provide satellite images from inside Russia. Soon after, he requested C.I.A. assistance in planning a clandestine mission to send HUR commandos into Russia to plant explosive devices at train depots used by the Russian military. If the Russian military sought to take more Ukrainian territory, Ukrainians could detonate the explosives to slow the Russian advance.

When the station chief briefed his superiors, they “lost their minds,” as one former official put it. Mr. Brennan, the C.I.A. director, called General Kondratiuk to make certain that mission was canceled and that Ukraine abided by the red lines forbidding lethal operations.

General Kondratiuk canceled the mission, but he also took a different lesson. “Going forward, we worked to not have discussions about these things with your guys,” he said.

Late that summer, Ukrainian spies discovered that Russian forces were deploying attack helicopters at an airfield on the Russian-occupied Crimean Peninsula, possibly to stage a surprise attack.

General Kondratiuk decided to send a team into Crimea to plant explosives at the airfield so they could be detonated if Russia moved to attack.

This time, he didn’t ask the C.I.A. for permission. He turned to Unit 2245, the commando force that received specialized military training from the C.I.A.’s elite paramilitary group, known as the Ground Department. The intent of the training was to teach defensive techniques, but C.I.A. officers understood that without their knowledge the Ukrainians could use the same techniques in offensive lethal operations.

At the time, the future head of Ukraine’s military intelligence agency, General Budanov, was a rising star in Unit 2245. He was known for daring operations behind enemy lines and had deep ties to the C.I.A. The agency had trained him and also taken the extraordinary step of sending him for rehabilitation to Walter Reed National Military Medical Center in Maryland after he was shot in the right arm during fighting in the Donbas.

Disguised in Russian uniforms, then-Lt. Col. Budanov led commandos across a narrow gulf in inflatable speedboats, landing at night in Crimea.

But an elite Russian commando unit was waiting for them. The Ukrainians fought back, killing several Russian fighters, including the son of a general, before retreating to the shoreline, plunging into the sea and swimming for hours to Ukrainian-controlled territory.

It was a disaster. In a public address, President Putin accused the Ukrainians of plotting a terrorist attack and promised to avenge the deaths of the Russian fighters.

“There is no doubt that we will not let these things pass,” he said.

In Washington, the Obama White House was livid. Joseph R. Biden Jr., then the vice president and a champion of assistance to Ukraine, called Ukraine’s president to angrily complain.

“It causes a gigantic problem,” Mr. Biden said in the call, a recording of which was leaked and published online. “All I’m telling you as a friend is that my making arguments here is a hell of a lot harder now.”

Some of Mr. Obama’s advisers wanted to shut the C.I.A. program down, but Mr. Brennan persuaded them that doing so would be self-defeating, given the relationship was starting to produce intelligence on the Russians as the C.I.A. was investigating Russian election meddling.

Mr. Brennan got on the phone with General Kondratiuk to again emphasize the red lines.

The general was upset. “This is our country,” he responded, according to a colleague. “It’s our war, and we’ve got to fight.”

The blowback from Washington cost General Kondratiuk his job. But Ukraine didn’t back down.

One day after General Kondratiuk was removed, a mysterious explosion in the Russian-occupied city of Donetsk, in eastern Ukraine, ripped through an elevator carrying a senior Russian separatist commander named Arsen Pavlov, known by his nom de guerre, Motorola.

The C.I.A. soon learned that the assassins were members of the Fifth Directorate, the spy group that received C.I.A. training. Ukraine’s domestic intelligence agency had even handed out commemorative patches to those involved, each one stitched with the word “Lift,” the British term for an elevator.

Again, some of Mr. Obama’s advisers were furious, but they were lame ducks — the presidential election pitting Donald J. Trump against Hillary Rodham Clinton was three weeks away — and the assassinations continued.

A team of Ukrainian agents set up an unmanned, shoulder-fired rocket launcher in a building in the occupied territories. It was directly across from the office of a rebel commander named Mikhail Tolstykh, better known as Givi. Using a remote trigger, they fired the launcher as soon as Givi entered his office, killing him, according to U.S. and Ukrainian officials.

A shadow war was now in overdrive. The Russians used a car bomb to assassinate the head of Unit 2245, the elite Ukrainian commando force. The commander, Col. Maksim Shapoval, was on his way to meeting with C.I.A. officers in Kyiv when his car exploded.

At the colonel’s wake, the U.S. ambassador to Ukraine, Marie Yovanovitch, stood in mourning beside the C.I.A. station chief. Later, C.I.A. officers and their Ukrainian counterparts toasted Colonel Shapoval with whiskey shots.

“For all of us,” General Kondratiuk said, “it was a blow.”

Tiptoeing Around Trump

The election of Mr. Trump in November 2016 put the Ukrainians and their C.I.A. partners on edge.

Mr. Trump praised Mr. Putin and dismissed Russia’s role in election interference. He was suspicious of Ukraine and later tried to pressure its president, Volodymyr Zelensky, to investigate his Democratic rival, Mr. Biden, resulting in Mr. Trump’s first impeachment.

But whatever Mr. Trump said and did, his administration often went in the other direction. This is because Mr. Trump had put Russia hawks in key positions, including Mike Pompeo as C.I.A. director and John Bolton as national security adviser. They visited Kyiv to underline their full support for the secret partnership, which expanded to include more specialized training programs and the building of additional secret bases.

The base in the forest grew to include a new command center and barracks, and swelled from 80 to 800 Ukrainian intelligence officers. Preventing Russia from interfering in future U.S. elections was a top C.I.A. priority during this period, and Ukrainian and American intelligence officers joined forces to probe the computer systems of Russia’s intelligence agencies to identify operatives trying to manipulate voters.

In one joint operation, a HUR team duped an officer from Russia’s military intelligence service into providing information that allowed the C.I.A. to connect Russia’s government to the so-called Fancy Bear hacking group, which had been linked to election interference efforts in a number of countries.

General Budanov, whom Mr. Zelensky tapped to lead the HUR in 2020, said of the partnership: “It only strengthened. It grew systematically. The cooperation expanded to additional spheres and became more large-scale.”

The relationship was so successful that the C.I.A. wanted to replicate it with other European intelligence services that shared a focus in countering Russia.

The head of Russia House, the C.I.A. department overseeing operations against Russia, organized a secret meeting at The Hague. There, representatives from the C.I.A., Britain’s MI6, the HUR, the Dutch service (a critical intelligence ally) and other agencies agreed to start pooling together more of their intelligence on Russia.

The result was a secret coalition against Russia — and the Ukrainians were vital members of it.

March to War

In March 2021, the Russian military started massing troops along the border with Ukraine. As the months passed, and more troops encircled the country, the question was whether Mr. Putin was making a feint or preparing for war.

That November, and in the weeks that followed, the C.I.A. and MI6 delivered a unified message to their Ukrainian partners: Russia was preparing for a full-scale invasion to decapitate the government and install a puppet in Kyiv who would do the Kremlin’s bidding.

U.S. and British intelligence agencies had intercepts that Ukrainian intelligence agencies did not have access to, according to U.S. officials. The new intelligence listed the names of Ukrainian officials whom the Russians were planning to kill or capture, as well as the Ukrainians the Kremlin hoped to install in power.

President Zelensky and some of his top advisers appeared unconvinced, even after Mr. Burns, the C.I.A. director, rushed to Kyiv in January 2022 to brief them.

As the Russian invasion neared, C.I.A. and MI6 officers made final visits in Kyiv with their Ukrainian peers. One of the M16 officers teared up in front of the Ukrainians, out of concern that the Russians would kill them.

At Mr. Burns’s urging, a small group of C.I.A. officers were exempted from the broader U.S. evacuation and were relocated to a hotel complex in western Ukraine. They didn’t want to desert their partners.

No Endgame

After Mr. Putin launched the invasion on Feb. 24, 2022, the C.I.A. officers at the hotel were the only U.S. government presence on the ground. Every day at the hotel, they met with their Ukrainian contacts to pass information. The old handcuffs were off, and the Biden White House authorized spy agencies to provide intelligence support for lethal operations against Russian forces on Ukrainian soil.

Often, the C.I.A. briefings contained shockingly specific details.

On March 3, 2022 — the eighth day of the war — the C.I.A. team gave a precise overview of Russian plans for the coming two weeks. The Russians would open a humanitarian corridor out of the besieged city of Mariupol that same day, and then open fire on the Ukrainians who used it.

The Russians planned to encircle the strategic port city of Odesa, according to the C.I.A., but a storm delayed the assault and the Russians never took the city. Then, on March 10, the Russians intended to bombard six Ukrainian cities, and had already entered coordinates into cruise missiles for those strikes.

The Russians also were trying to assassinate top Ukrainian officials, including Mr. Zelensky. In at least one case, the C.I.A. shared intelligence with Ukraine’s domestic agency that helped disrupt a plot against the president, according to a senior Ukrainian official.

When the Russian assault on Kyiv had stalled, the C.I.A. station chief rejoiced and told his Ukrainian counterparts that they were “punching the Russians in the face,” according to a Ukrainian officer who was in the room.

Within weeks, the C.I.A. had returned to Kyiv, and the agency sent in scores of new officers to help the Ukrainians. A senior U.S. official said of the C.I.A.’s sizable presence, “Are they pulling triggers? No. Are they helping with targeting? Absolutely.”

Some of the C.I.A. officers were deployed to Ukrainian bases. They reviewed lists of potential Russian targets that the Ukrainians were preparing to strike, comparing the information that the Ukrainians had with U.S. intelligence to ensure that it was accurate.

Before the invasion, the C.I.A. and MI6 had trained their Ukrainian counterparts on recruiting sources, and building clandestine and partisan networks. In the southern Kherson region, which was occupied by Russia in the first weeks of the war, those partisan networks sprang into action, according to General Kondratiuk, assassinating local collaborators and helping Ukrainian forces target Russian positions.

In July 2022, Ukrainian spies saw Russian convoys preparing to cross a strategic bridge across the Dnipro river and notified MI6. British and American intelligence officers then quickly verified the Ukrainian intelligence, using real-time satellite imagery. MI6 relayed the confirmation, and the Ukrainian military opened fire with rockets, destroying the convoys.

At the underground bunker, General Dvoretskiy said a German antiaircraft system now defends against Russian attacks. An air-filtration system guards against chemical weapons and a dedicated power system is available, if the power grid goes down.

The question that some Ukrainian intelligence officers are now asking their American counterparts — as Republicans in the House weigh whether to cut off billions of dollars in aid — is whether the C.I.A. will abandon them. “It happened in Afghanistan before and now it’s going to happen in Ukraine,” a senior Ukrainian officer said.

Referring to Mr. Burns’s visit to Kyiv last week, a C.I.A. official said, “We have demonstrated a clear commitment to Ukraine over many years and this visit was another strong signal that the U.S. commitment will continue.”

The C.I.A. and the HUR have built two other secret bases to intercept Russian communications, and combined with the 12 forward operating bases, which General Kondratiuk says are still operational, the HUR now collects and produces more intelligence than at any time in the war — much of which it shares with the C.I.A.

“You can’t get information like this anywhere — except here, and now,” General Dvoretskiy said.