Ukraine’s president, Volodymyr Zelensky, is making a rare trip to Washington this week, pleading his case for American military aid, something which has long been a lifeline for his country but is now increasingly in doubt. Julian Barnes, who covers international security for The Times, explains what has brought Ukraine to the most perilous point since the war began nearly two years ago. Guest: Julian E. Barnes, a correspondent covering the U.S. intelligence agencies and international security for The New York Times.