President Volodymyr Zelensky of Ukraine was already facing a daunting week as foreign officials gathered in Europe for talks about his country’s future.

The Trump administration was demanding $500 billion in Ukrainian mineral rights, it canceled Ukraine’s exemption from U.S. tariffs on steel, and a leading American skeptic of military assistance for Kyiv, Vice President JD Vance, was on his way to Europe for a meeting with the Ukrainian leader.

But on Wednesday, things went from bad to worse. Mr. Trump’s defense secretary delivered a harsh assessment of Ukraine’s prospects in its war with Russia. Then Mr. Trump announced that he had spoken with President Vladimir V. Putin of Russia, a call Mr. Trump characterized as the opening of talks to end the war — with no clear role for Mr. Zelensky.

The phone call also spelled the end of American efforts to isolate Russia diplomatically after its full-scale invasion of Ukraine nearly three years ago

“He’s on his heels geopolitically,” Cliff Kupchan, chairman of Eurasia Group, a risk analysis firm based in Washington, said of Mr. Zelensky.

Mr. Trump’s actions in the last two days — which also included a prisoner swap with the Kremlin that freed an American teacher — signaled a thawing relationship between the United States and Russia that could favor Mr. Putin in a peace deal while leaving Ukraine on the sidelines. Mr. Trump also called the Ukrainian leader on Wednesday, but in a social media post he did not mention how, or if, Mr. Zelensky would figure in peace talks.