President Volodymyr Zelensky returned to Ukraine on Monday after a whirlwind diplomatic mission that included both humiliation, by President Trump, and a warm embrace, from European leaders. He vowed to use all diplomatic avenues to pursue an end to Ukraine’s war with Russia, but acknowledged there was “a long way to go.”

Russia has given no indication that it will accept any terms other than Ukrainian capitulation and permanent conquest of a large swath of Ukraine — and Mr. Trump makes clearer by the day that his intent is to stand with Moscow.

Ukrainians have insisted they will not lay down their arms unless they receive security guarantees, supported by the United States, that would prevent the Kremlin from regrouping and attacking again.

After a disastrous meeting with Mr. Trump on Friday, in which the American president and Vice President JD Vance berated him for being ungrateful, Mr. Zelensky received a show of support from Europe’s democracies on Sunday, who pledged to work with Ukraine to come up with a peace plan that it could then present to the United States.

Mr. Zelensky himself said American buy-in for a peace plan was important and seemed to go further in his efforts to smooth things over with the White House. “We are grateful for all the support we have received from the United States,” he said in his address to the nation Sunday night. “There hasn’t been a single day when we haven’t felt grateful.”

“There will be diplomacy for peace,” Mr. Zelensky said. “And for the sake of all of us standing together — Ukraine, the whole of Europe, and necessarily America.”

But the Ukrainian leader faces a monumental challenge in repairing his relationship with Mr. Trump and his advisers.

In a series of coordinated interviews on American television on Sunday, top Trump administration officials assailed the Ukrainian leader, often in remarkably personal terms.

National security adviser Mike Waltz compared him to “an ex-girlfriend that wants to argue everything that you said nine years ago, rather than moving the relationship forward.”

The new director of National Intelligence, Tulsi Gabbard, questioned whether Ukraine and the United States really share common values and extended her critique to European countries who rallied around Mr. Zelensky, saying, they “don’t stand with us around these fundamental values of freedom.”

When asked by the host of Fox News Sunday if Russia stood for the same values as Americans, she said “that’s not really what we’re talking about here.”

After Mr. Zelensky left the White House, Mr. Trump said he could return when he is “ready for peace.”

But Mr. Trump has said little about any concessions from the Kremlin as he aligns himself ever more closely with President Vladimir V. Putin of Russia.

“We should spend less time worrying about Putin, and more time worrying about migrant rape gangs, drug lords, murderers, and people from mental institutions entering our Country,” he wrote in a post on social media. “So that we don’t end up like Europe!”

The Oval Office meeting fed into propaganda from the Kremlin, which added to the pile-on on Monday.

The meeting showed that “the Kyiv regime and Zelensky don’t want peace, they want continuation of the war,” the Kremlin spokesman, Dmitri S. Peskov, told reporters on Monday. His comments connoted a tightening bond between Moscow and Washington as they falsely portray Ukraine as the aggressor, unwilling to make peace. Attempts by Washington and Moscow “will clearly not be enough” to end the fighting, Mr. Peskov said. “An important element is missing.”

He implied that Russia could now push for a harder bargain than it did during the failed peace talks at the start of the war, given Russia’s military gains. “Since then, 2 ½ years later, the situation has changed,” Mr. Peskov said. “Only the blind can’t see that or the deaf not wanting to hear that.”

Mr. Zelensky sought to defend himself from being painted as the obstacle to peace — a criticism many Ukrainians find hard to understand given that their country that has been under fierce attack for three years.

Ukrainians almost universally want peace, just not peace at any cost. “We need peace, not endless war,” Mr. Zelensky said once again as he returned to Ukraine.

But bitter experience has Ukrainians worried that a cease-fire without security guarantees would only provide a brief respite for Russian forces to regroup and attack again. They point to the fact that Ukraine has been fighting Russia in the eastern Donbas region since 2014 and that Mr. Putin has violated multiple peace accords aimed at ending the violence there. The Russian leader also claimed he had no intention of mounting a broader invasion of Ukraine right up until his tanks rolled across the border three years ago.

Mr. Zelensky’s insistence on pushing for security guarantees was one of the things that apparently angered Mr. Trump.

As Mr. Zelensky now works with European leaders to come up with a peace plan, he said once again that there are some fundamental principals not open to negotiation.

“We need to proceed from the understanding of international law,” he said during a meeting with reporters in London. “We don’t want anything that doesn’t belong to us, but when you occupy something or when you break the law, everything will come back to you,” he said.

He emphasized that Ukraine will never recognize the occupied territories as Russian: “For us, this will be temporary occupations.”

Russia, he said, would need to take concrete actions before any deal.

“The cease-fire must begin with the exchange of prisoners and the return of children,” his office wrote in a statement. “This would be a step to demonstrate Russia’s genuine intent for peace.”

The International Criminal Court has issued an arrest warrant for Mr. Putin and accused him of war crimes, based on Russia’s abduction and deportation of thousands of Ukrainian children during the war.

The French have also suggested a staged process, perhaps with a truce regarding strikes on energy infrastructure by both sides.

For now, the fighting rages on as violently as ever.

Since Mr. Trump spoke with Mr. Putin two weeks ago, there has been no letup Russian attacks. Dozens of Ukrainian civilians have been killed in the past two weeks, according to Ukrainian officials, as Russia continues to launch nightly drone and missile attacks.

At the same time, Ukraine has kept up its campaign targeting Russian oil and gas refineries, hoping to deepen the economic pressure on Moscow.

In one attack, drones reportedly took aim at the Ufimsky refinery plant, more than 800 miles from the nearest Ukrainian-held territory. It was not possible to immediately assess the impact of the attack.

Hopeful that America’s turn against Kyiv could do for him what his military has failed to do, Mr. Putin has stuck to his maximalist goals in public comments in recent days.

Those include a desire to take control over wide swaths of land his forces do not yet occupy and which at the current pace of the Russian military’s creeping advances it would take many years to capture.

Anatoly Kurmanaev contributed reporting from Berlin.