Since the start of the war in Ukraine, no single figure has antagonized President Vladimir V. Putin of Russia as effectively or persistently as President Volodymyr Zelensky of Ukraine. His defiant videos and speeches have inspired the West into action and, by his own account, made him a target for Russian assassins. What is it about the comedian-turned-president and his rise to power that poses such a unique threat to Mr. Putin? Guest: Anton Troianovski, the Moscow bureau chief for The New York Times.