Gen. Oleksandr Syrskyi, who was replaced as commander in chief of Ukrainian forces late on Tuesday, was a flint-hard, highly decorated military leader and one of the most significant figures of Ukraine’s war against Russia. But his strengths early in the war and his unyielding personality may ultimately have led to his political downfall.

Renowned for his role in the defense of Kyiv, the capital, at the beginning of Russia’s full-scale invasion in February 2022, he gained the trust of President Volodymyr Zelensky and became commander in chief of Ukraine’s armed forces two years later, managing to hold the fight against Russia though grinding war with heavy armor and troops.

His departure signals the end of an era — a change of the Soviet-trained military old guard to a younger generation of commanders who have embraced technological innovation and collaborative leadership. The general oversaw an expansion of drone and robot warfare on the battlefield, but commanders and their supporters in the forefront of the fight have urged even faster development as key to defeating a powerful enemy.

His removal came after a dramatic disagreement with the former defense minister, Mykhailo Fedorov, who drove Ukraine’s innovative drone warfare. Mr. Fedorov, who complained that General Syrskyi was blocking the signing of military contracts for drones, was fired, setting off protests within the military and in the streets — with many of them demanding that General Syrskyi step down.

The confrontation brought an end to General Syrskyi’s four-decade military career, which began in a Soviet military academy and ultimately won him some of Ukraine’s top military awards, including the “Hero of Ukraine,” the highest honor.

He commanded crucial battles and was frequently pictured briefing the president at the front, which made him a public name. But he was often criticized for being a hard taskmaster and a micromanager who wanted to control every aspect of the armed forces. Critics complained of Soviet-style recklessness with soldiers’ lives and of what they saw as his refusal to withdraw troops from positions long seen as untenable.

General Syrskyi defended himself on Wednesday. “I have witnessed this war since its first day in 2014, lived my life at the front and know the price of every kilometer of Ukrainian land that has been liberated,” he said on social media as he announced he was stepping down.

“I am handing over to my successor an army that is not merely holding the line but is in a state of offensive — with the initiative, with the structure and with people who know how to strike the enemy.”

Born in 1965 in the village of Novinki, east of Moscow, General Syrskyi grew up in Kharkiv in Ukraine, where his father served in the Soviet Army. A star pupil, he was picked to study engineering at an elite military academy in Moscow before returning to Ukraine and joining the military.

He climbed the ranks and in 2014, he played a key role as Russian forces intervened in support of a separatist movement in eastern Ukraine.

By the time the Russian invasion began, he was one of the most experienced generals in the army. When Mr. Zelensky was first elected, General Syrskyi met the new president during an early visit to a military training ground and quickly earned his trust.

Hanna Maliar, a former deputy defense minister who worked closely with General Syrskyi from 2021 to 2023, said in an interview that he worked 18-hour days, kept physically fit and never took weekends off.

She described him as a man of charisma but also of immense calm in a crisis, which drew people to rely on him in the first days of the Russian invasion in February 2022. He had proposed that someone be assigned the job of defending Kyiv and offered himself up two weeks before the Russian invasion, Ms. Maliar said.

A chess player who always kept a chess board close at hand in his headquarters, he was a meticulous planner, she said. As he prepared to defend Kyiv, he pulled out documents from the archives on the World War II battles in the region and planned the defense of the city by walking around the old German fortifications, she said.

“His personal achievement is this preparation,” she said.

“He thinks not in a standard way, employing tricks and movements that are totally unexpected,” she added.

After seeing off the Russian threat to Kyiv, General Syrskyi was sent to the eastern front to bolster Ukrainian defenses as Russian forces turned their focus to seizing the eastern provinces of the Donbas region.

By late summer of 2022, he saw a weakness in Russian defenses and pounced, precipitating a sweeping counteroffensive that pushed back Russian forces and regained a broad stretch of Ukrainian territory in northeastern Ukraine.

In 2023, he oversaw the bloody, monthslong defense of the city of Bakhmut. The government celebrated it as a heroic battle, but soldiers and analysts criticized it as a wasted effort for little strategic gain that drew resources from the main counteroffensive in southern Ukraine that summer.

That larger counteroffensive, overseen by Gen. Valery Zaluzhny, failed, and in February 2024 Mr. Zelensky replaced him with General Syrskyi as commander in chief.

Succeeding General Zaluzhny, a hugely well liked and empathetic leader, was unpopular at the time. But then, in the summer of 2024, General Syrskyi mounted another surprise offensive, ordering an incursion into the Russian region of Kursk to pre-empt a Russian assault.

Taking the attack into Russia boosted morale at home and diverted Russian forces for several months, but when Russia gathered a response, Ukraine lost some of its best fighters and equipment at a time when both were in increasingly short supply. As the war ground on, Russia steadily ate up territory and heaped misery on Ukrainians with withering strikes on infrastructure through a bitterly cold winter.

General Syrskyi gained a nickname, the butcher, among soldiers. Ms. Maliar, who has remained close to General Syrskyi since she left the defense ministry and returned to an academic job, said the name was a misnomer and that when planning operations, he was always intent on keeping casualties low.

The defense of Bakhmut was merited, she said, because it allowed Ukraine to destroy Wagner, the Russian paramilitary force, which she said would have done much greater damage to Ukrainian defenses if it had survived.

But the criticism over General Syrskyi’s leadership reached a peak in recent weeks.

Bohdan Krotevych, a former lieutenant colonel in the Ukrainian National Guard, had for several years complained of General Syrskyi’s frequent change of units to fill gaps along Ukraine’s 800-mile front line.

“Another major problem was micromanagement,” Mr. Krotevych said. “He sidelined promising officers, promoted loyalists and diverted resources away from those who were capable of delivering results.”

Ms. Maliar responded, “It’s not micromanagement; it’s about keeping your finger on the pulse.”

Some commanders voiced support in recent days, saying that General Syrskyi was the only commander with the breadth of experience to oversee military operations around the whole country.

Oleksandr Chubko and Kamila Hrabchuk contributed reporting.