For months, President Trump has been threatening to simply walk away from the frustrating negotiations for a cease-fire between Russia and Ukraine.

After a phone call on Monday between Mr. Trump and President Vladimir V. Putin of Russia, that appears to be exactly what the American president is doing. The deeper question now is whether he is also abandoning America’s three-year-long project to support Ukraine, a nascent democracy that he has frequently blamed for being illegally invaded.

Mr. Trump told President Volodymyr Zelensky of Ukraine and other European leaders after his call with Mr. Putin that Russia and Ukraine would have to find a solution to the war themselves, just days after saying that only he and Mr. Putin had the power to broker a deal. And he backed away from his own threats to join a European pressure campaign that would include new sanctions on Russia, according to six officials who were familiar with the discussion. They spoke on condition of anonymity to describe a private conversation.

Their account sheds light on Mr. Trump’s decision to throw up his hands when it comes to a peace process that he had previously promised to resolve in just 24 hours. And, unless he again reverses course, Monday’s developments left Mr. Putin with exactly what he wanted: not only an end to American pressure, but the creation of a deep fissure inside the North Atlantic Treaty Organization, between the Americans and their traditional European allies, who say they are going ahead with sanctions anyway.

To many, Mr. Trump’s decision was foretold — first by his fiery, televised encounter with Mr. Zelensky in the Oval Office, then by the resignation of the American ambassador in Kyiv.

“The policy since the beginning of the Trump administration has been to put pressure on the victim, Ukraine, rather than the aggressor, Russia,” Bridget A. Brink, the former ambassador and a longtime Foreign Service officer, wrote after leaving Kyiv last month. “Peace at any price is not peace at all — it is appeasement.”