ON A TRAIN TO KYIV FROM SUMY, Ukraine — President Zelensky is warning that unless his nation wins a drawn-out battle in a key eastern city, Russia could begin building international support for a deal that could require Ukraine to make unacceptable compromises. He also invited the leader of China, long aligned with Russia, to visit.

If Bakhmut fell to Russian forces, their president, Vladimir Putin, would “sell this victory to the West, to his society, to China, to Iran,” Mr. Zelensky said in an exclusive interview with the Associated Press.

“If he will feel some blood — smell that we are weak — he will push, push, push,” Mr. Zelensky said in English, which he used for virtually all of the interview.

The leader spoke to the AP aboard a train shuttling him across Ukraine, to cities near some of the fiercest fighting and others where his country’s forces have successfully repelled Russia’s invasion. The AP is the first news organization to travel extensively with Mr. Zelensky since the war began just more than a year ago.

Since then, Ukraine — backed by much of the West — has surprised the world with the strength of its resistance against the larger, better-equipped Russian military. Ukrainian forces have held their capital, Kyiv, and pushed Russia back from other strategically important areas.

Yet as the war enters its second year, Mr. Zelensky finds himself focused on keeping motivation high in both his military and the general Ukrainian population — particularly the millions who have fled abroad and those living in relative comfort and security far from the front lines.

Mr. Zelensky is also well aware that his country’s success has been in great part due to waves of international military support, particularly from America and Western Europe. But some in America — including Republican Donald Trump, the former American president and current 2024 candidate — have questioned whether Washington should continue to supply Ukraine with billions of dollars in military aid.

Mr. Trump’s likely Republican rival, Governor DeSantis, also suggested that defending Ukraine in a “territorial dispute” with Russia was not a significant U.S. national security priority. He later walked that statement back after facing criticism from other corners of the GOP.

Mr. Zelensky didn’t mention the names of Mr. Trump or any other Republican politicians — figures he might have to deal with if they prevail in 2024 elections — but he did say that he worries the war could be affected by shifting political forces in Washington.

“The United States really understands that if they stop helping us, we will not win,” he said in the interview. He sipped tea as he sat on a narrow bed in the cramped, unadorned sleeper cabin on a state railway train.

The president’s carefully calibrated railroad trip was a remarkable journey across land through a country at war. Mr. Zelensky, who has become a recognizable face across the world as he doggedly tells his side of the story to nation after nation, used the morale-building journey to carry his considerable clout to regions close to the front lines.

He traveled with a small cadre of advisers and a large group of heavily armed security officials dressed in battlefield fatigues. His destinations included ceremonies marking the one-year anniversary of the liberation of towns in the Sumy region and visits with troops stationed at front-line positions near Zaporizhzhia. Each visit was kept under wraps until after he departed.

Mr. Zelensky recently made a similar visit near Bakhmut, where Ukrainian and Russian forces have been locked for months in a grinding and bloody battle. While some Western military analysts have suggested that the city is not of significant strategic importance, Mr. Zelensky warned that a loss anywhere at this stage in the war could put Ukraine’s hard-fought momentum at risk.

“We can’t lose the steps because the war is a pie — pieces of victories. Small victories, small steps,” he said.

Mr. Zelensky’s comments were an acknowledgement that losing the 7-month-long battle for Bakhmut — the longest of the war thus far — would be more of a costly political defeat than a tactical one.

He predicted that the pressure from a defeat in Bakhmut would come quickly — both from the international community and within his own country. “Our society will feel tired,” he said. “Our society will push me to compromise with them.”

So far, Mr. Zelensky says he hasn’t felt that pressure. The international community has largely rallied around Ukraine following Russia’s February 24, 2022, invasion. In recent months, a parade of world leaders have visited Mr. Zelensky in Ukraine, most traveling on trains similar to the ones the president uses to crisscross the country.

In his AP interview, Mr. Zelensky extended an invitation to Ukraine to one notable and strategically important leader who has not made the journey — the Chinese president.

“We are ready to see him here,” he said. “I want to speak with him. I had contact with him before full-scale war. But during all this year, more than one year, I didn’t have.”

China, economically aligned and politically favorable toward Russia across many decades, has provided Mr. Putin diplomatic cover by staking out an official position of neutrality in the war.

Asked whether President Xi would accept an invitation from Mr. Zelensky — or whether one had been officially extended — the Chinese foreign ministry spokeswoman, Mao Ning, told reporters she had no information to give. She did say that Beijing maintains “communication with all parties concerned, including Ukraine.”

Mr. Xi visited Mr. Putin in Russia last week, raising the prospect that Beijing might be ready to provide Moscow with the weapons and ammunition it needs to refill its depleted stockpile. But Mr. Xi’s trip ended without any such announcement. Days later, Mr. Putin announced that he would be deploying tactical nuclear weapons to Belarus, which neighbors Russia and pushes the Kremlin’s nuclear stockpile closer to NATO territory.

Mr. Zelensky suggested Mr. Putin’s move was intended to distract from the lack of guarantees he received from China.

“What does it mean? It means that the visit was not good for Russia,” Mr. Zelensky speculated.

The president makes few predictions about the biggest question hanging over the war: how it will end. He expressed confidence, however, that his nation will prevail through a series of “small victories” and “small steps” against a “very big country, big enemy, big army” — but an army, he said, with “small hearts.”

And Ukraine itself? While Mr. Zelensky acknowledged that the war has “changed us,” he said that in the end, it has made his society stronger.

“It could’ve gone one way, to divide the country, or another way — to unite us,” he said. “I’m so thankful. I’m thankful to everybody — every single partner, our people, thank God, everybody — that we found this way in this critical moment for the nation. Finding this way was the thing that saved our nation, and we saved our land. We are together.”

By Julie Pace and Hanna Arhirova