KYIV — When Volodymyr Zelenskyy told his top commander, Gen. Valery Zaluzhny, on Monday that he would soon be dismissed, the Ukrainian president suggested a leadership change might help provide a refresh. The public is increasingly exhausted by the war, and aid from international partners has slowed, Zelenskyy said, according to a senior Ukrainian official familiar with their conversation.

But a swift, negative reaction in the military ranks, misgivings among some officials in Kyiv, and uncertainty in the West suggest Zelenskyy’s removal of the popular general could backfire — allowing Moscow to seize on the instability. It could also deliver a blow to morale among troops on the front lines, especially because there has been no public explanation for Zaluzhny’s expected dismissal.

“Only Russia wins in this situation,” said a current senior military official, who spoke on the condition of anonymity because he was not authorized to do so publicly. “This was very poorly communicated,” the military official added. “People needed to be prepared because Zaluzhny is very respected — not only among soldiers but civilians, too.”

In the conversation between Zelenskyy and Zaluzhny, which the first senior official described, speaking on the condition of anonymity to be candid, the general laid out some parting thoughts about the problems his successor will inherit — making clear why a quick improvement of Ukraine’s position on the battlefield is unlikely.

Any new commander will still be fighting a larger, better-armed force in a war that has reached an attritional, grinding stalemate. The new commander will still need a robust influx of additional soldiers, to replace losses and to match Russia’s plan to send 400,000 more troops into the fight. And Ukrainian forces will still need more of everything: weapons, ammunition, vehicles and other equipment.

Zaluzhny wasn’t angry about his firing, the senior official said. Indeed, the relationship between the two men had frayed over the course of nearly two years of war with Russia. They simply didn’t trust each other, U.S. and Ukrainian officials have said. Zelenskyy, in turn, listened carefully on Monday night.

There is still a chance that the president could reverse course. A formal decree making Zaluzhny’s removal official is expected to come this week, an official familiar with the matter said, but as of Thursday afternoon it had still not arrived.

On Monday, Zelenskyy’s spokesman, Serhiy Nykyforov, denied that the president had fired Zaluzhny, but he has not responded to an updated request for comment since then.

Perhaps the most serious disagreement in the rift between Zelenskyy and Zaluzhny was the general’s request to mobilize more soldiers.

Russia has more forces and weapons than Ukraine, and for Kyiv to gain on the battlefield, Zaluzhny told the president, it must mobilize at least as many people as Russia plans to — some 400,000, according to the senior official familiar with their meeting. Ukraine must also prepare for losses, which are expected to be comparable to last year’s.

Zaluzhny’s ultimate figure was close to 500,000, the official said. But Zelenskyy has pushed back on conscripting so many, in part, he said, because Ukraine lacks the money to pay them without significantly raising taxes for ordinary citizens. Such aggressive conscription would also be politically unpopular for the president, though military commanders say the need for it is elementary.

“There really aren’t enough people,” said a major who is commanding a unit in eastern Ukraine. “Even if it’s just defensive actions, there are still constant losses. Someone is sick, someone is discharged due to health conditions or someone is moved to a rear position. So there are fewer and fewer people willing to sit at the actual front line.

“Taking into account that the Russians have been conducting offensive actions for several months, mainly in eastern Ukraine, the need for people is critical,” the major said.

Whoever replaces Zaluzhny will almost certainly be close and more loyal to the president. The top contenders for the post are Col. Gen. Oleksandr Syrsky, the current ground forces commander, and Lt. Gen. Kyrylo Budanov, the military intelligence chief.

U.S. officials, however, had long sensed tension and distrust between Zelenskyy and Zaluzhny — in part because the president suspected his military chief had political ambitions. Zaluzhny, though charismatic, had a reputation for being bold and unafraid to speak his mind. That led to occasional spats with his Western counterparts, including Gen. Mark A. Milley, the former chairman of the Joint Chiefs of Staff, though overall they enjoyed a close relationship and at times spoke several times per week.

Syrsky, on the other hand, seemed more impressive and inspiring as a classic combat soldier, some officials said, with a clear grasp of the battlefield implications of what he viewed as political decisions.

Syrsky was also considered more accessible to some U.S. commanders. He built rapport with Gen. Christopher Cavoli, who as head of the U.S. European Command oversaw much of the Pentagon’s effort to train and equip Ukraine’s army. Meanwhile, during Ukraine’s summer counteroffensive, Cavoli could not reach Zaluzhny for weeks, though Zaluzhny maintained contact with Milley, people familiar with the matter said.

Within the ranks of the Ukrainian military, however, Syrsky is widely disliked. Some soldiers say his orders are unreasonable, at times sending men to their obvious deaths. Others disrespect him for removing popular commanders in favor of those more loyal to him.

Meanwhile, Budanov, with a background in special forces, has little experience as an army commander. Some military officials have suggested that even if Budanov is tapped to replace Zaluzhny, it would be Syrsky who would actually be commanding from behind the scenes.

Some soldiers and analysts said there was a danger that the next commander will be too deferential to Zelenskyy. Olena Tregub, a member of the Anti-Corruption Council of the Ukrainian Defense Ministry, said there was a risk of someone “who wants to say what Zelenskyy wants to hear.”

Soldiers voiced concern about their commander being co-opted. “Concentrating all power in one hand and one office is never a good idea,” said Vlad, a lieutenant who commands a mortar unit in eastern Ukraine. “This will create a kind of bubble over making crucial decisions at the front line and running the country in general.”

Part of the criticism of Zaluzhny by Zelenskyy’s team, according to a person familiar with the situation who spoke on the condition of anonymity, was the general’s repeated demands for more resources.

A White House $60 billion request for security assistance has stalled in Congress. On Thursday, European Union leaders approved more than $50 billion in aid to Kyiv — a rare bright spot for Ukraine, which has faced increasing Russian attacks. But the money is not expected to be a game changer on the battlefield.

Ukrainian and Western officials say Kyiv is likely to focus this year on defense — holding the line — rather than attempting to retake territory. And analysts and military personnel are deeply skeptical that any new commander could do more with less.

In Kyiv, there had been chatter for more than a year about the president’s unhappiness with Zaluzhny. And Zelenskyy had begun taking steps to show who was in charge. In August, he fired the heads of all Ukraine’s regional military recruitment offices, citing concerns about corruption.

That move rankled Zaluzhny, in part because it slowed Ukraine’s ongoing mobilization. Even six months later, not all of those positions have been filled.

A new commander will face other challenges. The average age of Ukraine’s troops, for example, is over 40, and some soldiers have been fighting for more than two years without much rest.

“Because there are not enough people in uniform, or at least not enough people in uniform in the right place, they can’t be brought out to be revitalized, reequipped and retrained, except in very small numbers,” said Gen. Richard Barrons, former commander of the British military’s Joint Forces Command.

“They’ve got this two-headed problem,” Barrons added. “The people at the front line are a bit weary in some cases. And they haven’t mobilized enough people to create a bigger army, which would allow them to do force rotation in a way most armies would want to do — nor to build the offensive force that they would need.”

A draft law in Ukraine’s parliament calls for lowering the minimum conscription age to 25 from 27, but Barrons suggested that Kyiv should aim to make its military even younger. “The fact is it is very unusual to have your wars fought by your dads,” he said.

Ukrainian lawmakers concede that mixed messaging about mobilization by Zelenskyy and Zaluzhny has sowed panic.

“They shouldn’t be pointing fingers at each other,” said Roman Kostenko, a member of parliament, who added that perhaps Ukraine should disclose its wartime casualties so people understand why more troops are needed.

The main reason Zelenskyy gave Zaluzhny for his dismissal was the need for a refresh. But many in Ukraine view the change as a step backward — a sign of the political infighting that was a Kyiv trademark before Russia’s invasion united the country.

“We had very, very strong unity on the top of the Ukrainian state at the beginning of the invasion, when everybody was united, everybody was protecting Ukraine,” Tregub said. “What is happening right now, it is, of course, concerning.”

Catherine Belton in London, Karen DeYoung in Washington, and Kostiantyn Khudov and Kamila Hrabchuk in Kyiv contributed to this report.

previous coverage

• Russia is wearing down Ukraine’s defenses while Zelenskyy fights with his top general
• Zelenskyy to oust Ukraine’s top general amid tension over new mobilization