Russia is unlikely to make significant territorial gains in Ukraine in the coming months as its poorly trained forces struggle to break through Ukrainian defenses that are now reinforced with Western munitions, U.S. officials say.

Through the spring and early summer, Russian troops tried to take territory outside the city of Kharkiv and renew a push in eastern Ukraine, to capitalize on their seizure of Avdiivka. Russia has suffered thousands of casualties in the drive while gaining little new territory.

Russia’s problems represent a significant change in the dynamic of the war, which had favored Moscow in recent months. Russian forces continue to inflict pain, but their incremental advances have been slowed by the Ukrainians’ hardened lines.

The months ahead will not be easy for Ukraine. But allied leaders gathering in Washington this week for the 75th anniversary of the founding of North Atlantic Treaty Organization can legitimately argue that their efforts to strengthen Ukraine are working.

“Ukrainian forces are stretched thin and face difficult months of fighting ahead, but a major Russian breakthrough is now unlikely,” said Michael Kofman, a senior fellow in the Russia and Eurasia program at the Carnegie Endowment for International Peace, who recently visited Ukraine.

Leaders at the summit are expected to promise new funding for Ukraine, announce plans for the alliance to coordinate weapons delivery and strengthen a promise to Kyiv that it will, eventually, become a full ally.

It is that last point that has become the focus of the war, more important even than reclaiming territory. While Ukrainian officials insist they are fighting to get their land back, growing numbers of U.S. officials believe that the war is instead primarily about Ukraine’s future in NATO and the European Union.

Looming over the summit are concerns about Russia’s acquisition of arms — particularly missiles, drones and the parts to build them — from Iran, North Korea and China.

And deep into the third year of a devastating war, there are real concerns about Ukraine’s ability to keep its infrastructure, including its electrical grid, functioning amid long-range Russian attacks.

But the biggest wild card of all may be U.S. policy toward Ukraine after the presidential election this fall.

While Russia is not in a position to seize large parts of Ukraine, the prospects of Kyiv retaking more land from the invading army are also waning. Prodded by American advisers, Ukraine is focused on building up its defenses and striking deep behind Russian lines.

Eric Ciaramella, a former intelligence official who is now an expert on Ukraine working with Mr. Kofman at the Carnegie Endowment for International Peace, said it had become clear over the past 18 months that neither Russia nor Ukraine “possesses the capabilities to significantly change the battle lines.”

The United States, Mr. Ciaramella said, has always defined its strategic objective “as a Ukraine that is democratic, prosperous, European and secure.” The United States and its allies will need to make long-term investments to enable Ukraine to hold its lines, wear out Russia and do damage, according to Mr. Ciaramella and current U.S. officials.

“That’s still a highly unstable scenario,” Mr. Ciaramella said. “That’s why Western leaders also really need to focus on integrating Ukraine into European and trans-Atlantic security structures. “

The European Union agreed last month to begin membership negotiations with Ukraine, a critical step in the long accession process. While NATO is not yet ready to invite Ukraine to join, allied leaders are set to approve language this week that all but promises Kyiv that it will become part of the alliance.

The statement aims to avoid a repeat of what happened at last year’s summit in Vilnius, Lithuania, where leaders declared that “Ukraine’s future is NATO” but did not follow that up with any concrete invitation. Diplomats called the convoluted language a “word salad,” and President Volodymyr Zelensky of Ukraine angrily complained about the lack of a time frame for membership.

The possibility of Ukraine joining NATO seemed distant before Russia’s 2022 invasion. Allies were reluctant to provoke Russia or take on what seemed like a vast security commitment. Since then, Ukraine’s partnership with the United States, Britain and other European countries has grown stronger, and the West has poured billions of dollars into training and equipping the Ukrainian army.

Keeping Ukraine out of NATO has been an aim of President Vladimir V. Putin of Russia since he began the war, one that ironically his invasion has made more of a possibility. Peace talks in April 2022 broke down when Moscow insisted on neutrality for Ukraine and a veto over any outside military assistance.

Since then, Ukraine has become even more committed to integrating into Europe.

Russia seized the most pro-Russian parts of Ukraine in the first year of the war. American officials say privately that it will be all but impossible for Ukraine to win back all its territory, but that it can insist on more European integration if its performance on the battlefield is stronger.

Some officials say that even without formally winning back its land, Ukraine could still emerge a victor in the war by moving closer to NATO and Europe.

Officials interviewed for this article spoke on the condition of anonymity to discuss secret military and intelligence assessments, battlefield positions and sensitive diplomacy.

American officials acknowledge that Russia could make significant headway, if there is a big strategic shift, such as by expanding its military draft and training program.

Their predictions would also be undermined if the U.S. policy toward Ukraine and Russia changed.

Under the Biden administration, the United States has provided military advice, real-time intelligence and billions of dollars in weapons.

Former President Donald J. Trump has promised that if elected, he would begin peace negotiations between Russia and Ukraine. While he has not outlined the peace terms he would seek, a quick negotiation would probably force Ukraine to cede vast swaths of territory and give up its ambitions to join NATO.

But officials say demanding that negotiations begin now would be a mistake. About $61 billion in aid approved by Congress in May after months of wrangling is strengthening Ukrainian defenses and halting Russia’s territorial advance.

Throughout the war, U.S. intelligence agencies have been far more pessimistic about its outlook than the Pentagon, whose senior officers have been working closely with Ukraine’s military to help develop its strategy. But assessments across the U.S. government now appear to be more aligned when it comes to Russia’s prospects on the battlefield.

With a supply of electronic components from China, drones from Iran and missiles and artillery from North Korea, Russia has secured enough weapons to keep its army supplied.

But it lacks sufficient personnel to mount a significant breakthrough.