Ukrainian armed forces commander General Valeriy Zaluzhnyi, who was replaced on Thursday, became a national hero for repelling Moscow's invading forces two years ago but suffered battlefield setbacks as the war ground on.
The move ends intense speculation over his fate after reported frictions between him and President Volodymyr Zelenskiy, whose authority will be tested as he seeks to rally troops under a new army chief and change the dynamic of the war.
Ukrainian forces are struggling after a counteroffensive launched last June made little headway in the south and east, while Russian forces are inflicting small but costly defeats at several points along the 1,000-km (620-mile)front.
Western military and financial support is no longer guaranteed, leaving Kyiv more exposed to attacks by Russian drones and missiles that are sapping Ukrainian resources.
Given Zaluznhyi's popularity and proven ability as an inspiring commander, the fact that Zelenskiy is replacing him may reflect the desire for a new approach on the battlefield.
He also criticised state institutions for failing to push through unpopular legislation that would reform the way Ukrainians are mobilised to fight, amid a shortage of soldiers and growing exhaustion among those already serving.
A HERO TO MANY
Defying the odds, Ukraine's soldiers used stealth and speed to thwart Russia's advance on Kyiv in February, 2022, helping to ensure that, even now, Russian President Vladimir Putin remains a long way from conquering Ukraine.
As the war progressed, Zaluzhnyi's stock rose, and he won praise at home and abroad when his forces launched counteroffensives in the northeast and south that recaptured swathes of land and raised hopes of an unlikely victory.
A portrait of him smiling and flashing the peace sign was spray-painted on walls after the liberation of the southern city of Kherson, under the slogan "God and Zaluzhnyi are with us".
Since then, Ukraine's battlefield momentum has stalled, yet polling indicated that Zaluzhnyi was still trusted by 92% of Ukrainians late last year, significantly above Zelenskiy's 77%.
Reported frictions between the two men burst into the open in November after Zaluzhnyi was quoted by the Economist as saying the war as at a "stalemate", a gloomy assessment that jarred with Zelenskiy's more optimistic vision.
The 50-year-old four-star general, who rarely speaks in public but is occasionally shown on news bulletins poring over maps and addressing commanders in fatigues, said then too that better technology was the key to breaking the impasse.
The president's office rebuked him, and one of Zaluzhnyi's senior officers said he had been sacked by Zelenskiy over the general's head.
Were he to go into politics - though he has never voiced political ambitions - the "Iron General" could prove a formidable force.
THE BURLY 'VOLUNTEER'
Zaluzhnyi began his military training in the 1990s, after Ukraine gained independence from the Soviet Union, graduating with honours and rising up the ranks.
He got a taste of real conflict in 2014 when he served in an area of eastern Ukraine where Russian-backed militants had seized territory.
Tall and burly with cropped hair, Zaluzhnyi, whose military call sign is "Volunteer", has a reputation for having a good rapport with his subordinates and allowing local commanders to make their own decisions on the battlefield.
His warning in November that the war was sliding into an attritional phase that suited Russia was out of kilter with Kyiv's official rhetoric, but for many of his soldiers it was recognition of the painful reality on the battlefield.
Russia had been building up fortifications since late 2022 after suffering humiliating defeats in the Kharkiv and Kherson regions, with more recent Ukrainian advances thwarted.
Tens of thousands of soldiers have been killed and wounded on both sides, although there are no reliable official figures.
Ukraine desperately needs to replenish its overstretched and exhausted ranks but the government has been unable to amend call-up laws to help recruit up to half a million more soldiers.
Kyiv has also struggled to maintain Western support that has been vital to its war effort.
The United States has failed to deliver a hefty aid package it had promised, although, in a boost for Ukraine, the European Union agreed to extend $54 billion in new support, overcoming weeks of resistance from Hungary.
Even so, as the deadliest conflict in Europe since World War Two enters its third year, Zaluzhnyi's boots will be hard to fill.
