During President Trump’s first term, Ukraine worried that Mr. Trump might recognize Russian control over Crimea, the Ukrainian peninsula Moscow forcibly seized in early 2014.

As a candidate, Mr. Trump had said he would “take a look” at the matter, even though the Obama administration and America’s Western allies had rejected Russia’s annexation of the strategic territory. Mr. Trump even mused that “the people of Crimea, from what I’ve heard, would rather be with Russia.”

But Mr. Trump never followed through and even doubled down against Moscow. In July 2018, Secretary of State Mike Pompeo issued an official “Crimea Declaration” pledging that the nonrecognition policy would remain “until Ukraine’s territorial integrity is restored.”

Now, in his effort to negotiate an end to the war between Russia and Ukraine, Mr. Trump is prepared to walk away from that declaration — and more than a decade of American policy.

A new peace proposal the Trump administration offered in London on Wednesday would include U.S. recognition that Crimea is part of Russia, American and European officials said.

Daniel Fried, a former diplomat with extensive experience with Ukraine and Russia, called it the worst element of the Trump proposal, which is widely seen as strongly favoring Moscow’s position in several ways.