Ukrainian President Volodymyr Zelensky was on an overnight train and in a gloomy mood when he decided to take the gloves off with President Trump.

News reached Zelensky and his advisers on the train from Kyiv to Poland late on Tuesday that Russian missiles had pounded Kyiv without the usual U.S. warning of a missile launch for the second day in a row, according to a person present.

In recent days, Trump had eased energy sanctions on Moscow, praised Russian President Vladimir Putin, piled pressure on Ukraine and called for Zelensky’s removal. The Ukrainian president feared more concessions to Moscow were coming, the person said.

For 18 months, since a disastrous Oval Office meeting with Trump early in his presidency, Zelensky had been trying to court him. Now he signaled to those around him that plain speaking was needed, the person said.

Zelensky, traveling to meet European allies in London and Brussels, decided to publicly criticize Trump’s approach, despite the risk of angering him further. He wanted to send a message to the public in the U.S. and Europe: Ukraine would defend itself however it needed to, and it wasn’t to blame for Russia’s invasion or for high global fuel prices.

The Ukrainian leader used scheduled interviews with British media on Wednesday to criticize Trump’s expressed preference for dealing with Putin and moves to relax economic sanctions on Moscow. “I find Zelensky to be more difficult to deal with than Putin,” Trump said this week.

Putin “is a real evil of today,” Zelensky told the Financial Times. “How can Trump be friends with him? I think that it’s a mistake. It’s his choice again and again.”

“Americans are on our side,” he said, appealing to the country beyond the White House. “I know that the majority of the United States supports Ukraine.”Zelensky’s pushback follows the arc of other European allies’ dealings with the American president since his return to the White House. A series of clashes over trade, the Russia-Ukraine war, the future of the North Atlantic Treaty Organization, the U.S. war with Iran and threats to seize the Danish territory of Greenland have led even once-close political allies in European capitals to distance themselves from Trump this year.

“I don’t know why the U.S. president behaves this way towards his own allies,” Italian Prime Minister Giorgia Meloni said in June when he made disparaging remarks about her. “It’s a pity that he doesn’t have the same determination with the enemies of the West, with the enemies of the United States, with whose leadership he shows himself to be much more accommodating.”Like Meloni and others in Europe, Zelensky risks further blowback from Trump for criticizing him. But the stakes for Ukraine are far higher.

Europe’s poorest country, locked in the fifth year of a war of survival against Russia’s attempted conquest, receives valuable U.S. support in three areas: intelligence-sharing, Starlink contracts, and sales of U.S. weapons and ammunition to Europe for use in Ukraine.

Trump cut virtually all other U.S. support for Ukraine in early 2025. Europe has since provided the bulk of Ukraine’s military and financial aid, but it can only partially replace American satellite and intelligence capabilities. Washington is also a crucial part of the Western sanctions regime that keeps economic pressure on Russia.

That sanctions policy is threatening to fray. Trump this month announced a deal with Putin to supply diesel to the U.S. and global markets, saying it would help bring down diesel prices. Then, on Friday, he waived most sanctions against Russia that were set to be levied under a bipartisan bill he signed last month.

The moves drew bipartisan criticism for weakening economic pressure on Moscow. Energy experts say the announced amounts of Russian diesel are too small to make much difference and the U.S.-Iran war has been the dominant driver of high global fuel prices this year.

Earlier on Oct. 9, just before Trump announced the diesel deal, Zelensky had held amicable discussions in Kyiv with a group of U.S. lawmakers who support Ukraine, led by Sen. Roger Wicker (R., Miss.), chairman of the Senate Armed Services Committee.

Republicans in the delegation had floated an idea with Ukrainian officials that the U.S. could give Ukraine access to Starlink up to around 200 miles deep inside Russia, so that its drones could hunt Russian ballistic-missile launchers. Ukraine can’t currently use Starlink inside Russia. In return, Kyiv could stop bombing Russian refineries, they said, according to Sen. Kevin Cramer (R., N.D.). Zelensky agreed it was a good idea, he said.

In Miami on the same day, however, White House negotiators Steve Witkoff and Jared Kushner pressed Ukrainian officials to stop hitting refineries with no quid pro quo, according to several participants in the meeting. The Americans repeatedly said they were giving Kyiv valuable intelligence, which Ukrainian and European officials present read as an implicit warning that it could be curtailed. U.S. officials deny any threat was made.

Trump had complained to Zelensky about Ukraine’s refinery strikes for several weeks, according to people familiar with their conversations, blaming them for high fuel prices that Republicans fear could hurt them in the midterm congressional elections in early November.

Zelensky replied that targeting Russia’s energy revenues, which finance its war machine, was a vital pillar of Ukraine’s strategy for forcing Putin to end the war, these people say. Trump, who once said Ukraine had no cards to play, was telling it to give up its strongest card.

Instead, Zelensky proposed options for a deal. Ukraine could halt refinery strikes if Russia stopped bombing Ukraine’s own energy infrastructure; or if the U.S. delivered more Patriot interceptors to protect Kyiv against ballistic-missile attacks this winter.

Putin rejected a mutual energy ceasefire and the White House didn’t agree to send more than a trickle of Patriots, which have been in short supply since the Iran war. Trump grew increasingly angry with Zelensky about Ukraine’s continuing refinery strikes. “He better damn well stop,” he said on Oct. 10.

Ukrainian officials said senior U.S. military leaders called their counterparts in Kyiv and warned them that intelligence cooperation was at risk if the refinery strikes continued. The U.S. Defense Department didn’t immediately respond to a request for comment. U.S. officials say intelligence-sharing continues and that undetected Russian launches were due to bad weather.

When Trump announced an “energy ceasefire” last Sunday, surprising both Kyiv and Moscow, Zelensky ordered a pause in the strikes. Russia continued to bomb power plants in Ukraine.

“That’s it,” Zelensky told the Financial Times, as Ukraine ended its pause after three days. Trump had said nothing about Russia’s heavy attacks on Ukraine’s energy infrastructure and killing of civilians, he complained. “Putin is comfortable now.”

Trump hit back in an interview with Newsmax on Thursday. “Zelensky’s not a good guy for us to deal with,” he said. “He says he wants to settle but he doesn’t. No, I’m not a big fan of Zelensky.”Ukraine couldn’t stay silent over Trump’s diesel deal with Putin, Zelensky said on Friday as he joined a summit of European leaders in Brussels. Next, Trump would do a gas deal, and then lift sanctions on other business dealings with Russia, he said.

Inside the summit, Europe’s heads of government hugged Zelensky as he arrived. None of them asked him to tone down criticism of Trump, according to European officials.

Late Friday, Trump signed an order halting the implementation of most of the sanctions that were to be imposed on Russia under a bipartisan bill passed in September. The law, named for the late Sen. Lindsey Graham (R., S.C.), authorized heavy tariffs on third countries that buy Russian oil.

Early Saturday, after another heavy Russian bombardment of Ukraine’s energy infrastructure, Zelensky said on social-media platform X he regretted the decision “to completely disregard the Lindsey Graham Act, which was supposed to impose truly effective pressure on Russia to bring this war to an end.”“This Act could have helped enormously—and it still can,” he said.

Write to Marcus Walker at Marcus. Walker@wsj.com and Robbie Gramer at robbie.gramer@wsj.com