For nearly three years of the war in Ukraine, Washington’s rallying cry in backing a fight against a Russian invasion was “no negotiations about Ukraine without Ukraine.”

But when President Trump meets President Vladimir V. Putin of Russia in Alaska on Friday, the Ukrainians will not be there, barring any last-minute invitation. And Kyiv’s swift rejection of Mr. Trump’s declaration to reporters that he is already negotiating with the Russian leadership over what he vaguely called “land swaps,” with no mention of the security guarantees or arms supplies for Ukraine, underscores the enormous risks for the Ukrainians — and the political perils for Mr. Trump.

Ukraine’s fear for these past six months has been that Mr. Trump’s image of a “peace accord” is a deal struck directly between him and Mr. Putin — much as Franklin Roosevelt, Joseph Stalin and Winston Churchill divided up Europe at the Yalta conference in 1945. That meeting has become synonymous with historical debates over what can go wrong when great powers carve up the world, smaller powers suffer the consequences and free people find themselves cast under authoritarian rule.

Mr. Zelensky himself invited such comparisons in a speech to his people hours after Mr. Trump raised the specter of deciding Ukraine’s fate in a one-on-one meeting in Alaska, territory that was once part of the Russian empire. (While Mr. Putin has made clear that he regards Ukraine as rightful Russian territory dating back to the days of Peter the Great, the Russian leader has not called for the reversal of the $7.2 million sale of Alaska to the United States in 1867, during a period of financial distress for the empire.)

“Ukrainians will not give their land to the occupier,” Mr. Zelensky said, noting that the Ukrainian constitution prohibits such a deal.