President Trump on Monday did not rule out sending U.S. troops to Ukraine as part of a peacekeeping force if a potential peace deal between Kyiv and Moscow can be reached. “We’ll let you know that, maybe later today,” Mr. Trump said during an Oval Office meeting with Ukrainian President Volodymyr Zelenskyy. Mr. Trump’s refusal to rule out the idea of U.S. boots on the ground in Ukraine is a potential major shift in his policy towards Ukraine as the war-torn nation considers security guarantees to be essential for any peace agreement. Mr. Trump met with Russian President Vladimir Putin...