President Volodymyr Zelensky has announced the dismissal of Ukraine’s top commander, General Valerii Zaluzhnyi, in the biggest military shakeup since the start of Russia’s full-scale invasion almost two years ago.

The president’s move follows tensions between Zelensky and his hugely popular military chief after the failure of Ukraine’s much-vaunted counteroffensive, and with Ukraine facing a renewed Russian onslaught, manpower and ammunition shortages, and US aid stalled in Congress.

In a Telegram post sent shortly before the formal announcement, Zelensky said he held a meeting with Zaluzhnyi, and “discussed what kind of renewal the Armed Forces of Ukraine need.”

“The time for such a renewal is now,” Zelensky wrote.

Zaluzhnyi’s replacement will be Oleksandr Syrskyi, who since 2019 has served as the Commander of Ukrainian Land Forces.

Zaluzhnyi wrote on his Telegram channel on Thursday that “the tasks of 2022 are different from those of 2024.

“Therefore, everyone must change and adapt to the new realities as well. [We] have just met with the Supreme Commander-in-Chief. It was an important and serious conversation. It was decided that we need to change our approaches and strategy.”

Rumors of Zaluzhnyi’s dismissal began to swirl around Kyiv last week after he was called to a meeting at the president’s office and told he was being fired, two sources familiar with the matter told CNN. Zelensky’s office initially denied the rumors, but the move was confirmed on Thursday.

Zaluzhnyi – who had been appointed army chief by Zelensky in July 2021 – was offered a new position by the president, which he turned down, according to one of the sources. It remains unclear whether Zaluzhnyi has decided to remain involved with the military in some capacity.

Differences between the two men had been simmering for many months but appeared to grow wider towards the end of last year, after Zaluzhnyi said the war had reached a stalemate in a long essay and interview in The Economist magazine in November.

Writing after Ukraine’s counteroffensive was mostly rebuffed by heavily fortified Russian defenses, he warned that without a great technological leap forward “there will most likely be no deep and beautiful breakthrough,” but instead an equilibrium of devastating losses and destruction.

His remarks drew immediate criticism from Zelensky’s office, which said such commentary about the war only benefitted Russia.

More recently, the two leaders clashed over whether Ukraine needed a mass mobilization effort. The army chief had suggested up to half a million draftees were required, which Zelensky resisted.

The president told a press conference in December mobilization was a ‘highly sensitive’ issue and that he wanted to hear more arguments in favor before he felt fully ready to back the move.

“This is a very serious number,” Zelensky said. “It is a question about people, about justice, about defense capabilities. It is also a financial question.”

Writing exclusively for CNN last week as rumors mounted, Zaluzhnyi made clear his frustrations over the issue, referring to “the inability of state institutions in Ukraine to improve the manpower levels of our armed forces without the use of unpopular measures.” The remarks were highly unusual for a man generally reluctant to speak publicly.

A political gamble for Zelensky

When Russia launched its invasion in February 2022, many of Ukraine’s allies feared Kyiv would fall in just a few days and the rest of the country within weeks. But Ukraine’s troops, under Zaluzhnyi’s direction, were able to drive Moscow’s forces from the capital and later in the year managed to reclaim large parts of the southern and eastern territories occupied by Russia in the early weeks of the war.

Ukraine had hoped to drive Moscow’s forces back further in 2023 but battlefield success proved elusive.

Launched last June, Ukraine’s counteroffensive in particular aimed to push south towards the Sea of Azov, splitting Russia’s forces in two and cutting its land bridge to Crimea.

But Ukraine’s gains were modest. Its forces attempted to advance from Orikhiv towards Tokmak, but only made it as far as Robotyne, a little over 20 kilometers (12.4 miles) south. Russia still occupies around a fifth of Ukraine’s territory.

Since then, Ukraine has been put back on a defensive footing by a flurry of Russian offensives along much of the frontlines, with intense fighting reported in northeastern Kharkiv region and in Zaporizhzhia in the south. Russia has also renewed its aerial assaults on cities across the country, and Ukraine has warned that its air defenses risk being overwhelmed.

Zaluzhnyi’s firing is a political gamble for Zelensky. Despite the failure of Ukraine’s counteroffensive, the now-former military chief remains one of the most popular leaders in the country. A poll by the Kyiv Institute of Sociology found 88% of Ukrainians supported the general. Zelensky’s approval rating, though also high, was markedly lower at 62%.

Analysts have long speculated about whether Zaluzhnyi could one day emerge as a political rival to Zelensky in future elections, although the general has shown scant political ambition so far.

In his CNN article, he also suggested Ukraine’s leadership had not tackled problems in the defense industries which had led to production bottlenecks and ammunition shortages.

At the same time, Zaluzhnyi warned Ukraine was now having to “contend with a reduction in military support from key allies” as they have become ensnarled by their own political tensions and distracted by conflicts elsewhere.

He said the best way for Ukraine’s army to avoid being drawn into a “positional war,” in which fighting is conducted along permanent and fortified frontlines, is for Ukraine to “master” unmanned weapons systems – or drones – which Zaluzhnyi called the “central driver of this war.”

He will now be replaced by the 59-year-old Syrskyi, who began his soldiering career during the last years of the Soviet Union, training in Moscow.

With Ukraine’s independence in 1991, he rose through the ranks of the Ukrainian armed forces becoming a Major General in 2009. He played a prominent role in Ukraine’s fight against Russia’s invaders in the eastern Donbas region in 2014 and 2015.

Two years later he became the commander of all Ukrainian forces involved in the Anti-Terrorist Operation, as the conflict with Russia in eastern Ukraine became known.

Following Russia’s full-scale invasion in February 2022, Syrskyi led the defense of Kyiv, and then later that year commanded the successful counteroffensive in the region of Kharkiv, which saw Russian forces driven out of hundreds of settlements.

But the move may prove controversial among Ukraine’s forces. A Ukrainian soldier fighting in the east of the country told CNN he was unconvinced by the change at the top of the military.

“I have the impression Syrskyi is an adequate person. But I’m sorry that [outgoing army head] Zaluzhnyi was dismissed. I do not think it will get easier at the front. Syrskyi is close to the authorities, but he needs to be independent,” the first solider told CNN, speaking on condition of anonymity.

Another, a frontline commander, also serving in Ukraine’s east, was more critical of the appointment.

“You do not dismiss the Commander-in-Chief in the middle of a war. Nothing good will come of it. It is playing into the hands of the Russians,” the commander told CNN, also on condition of anonymity.