Feb. 28 was one of the darkest days for Ukraine since Russia’s invasion three years earlier. An Oval Office visit by President Volodymyr Zelensky meant to win favor with President Trump turned into a televised shouting match, prompting Mr. Trump to banish his guest from the White House without even serving him a planned lunch. Mr. Trump was already a deep skeptic of U.S. support for Ukraine. But after the disastrous meeting with Mr. Zelensky, he accelerated his diplomacy with President Vladimir V. Putin of Russia, drafting a peace plan to end the war in Ukraine that offered major concessions...