The House Democrats leading the impeachment inquiry of President Trump called their first witness: Kurt Volker, a top American diplomat involved in the negotiations with Ukraine. We look at what Mr. Volker’s testimony — and the text messages he turned over to Congress — revealed about the inquiry’s direction. Guest: Julian E. Barnes, who covers national security for The New York Times. For more information on today’s episode, visit nytimes.com/thedaily.  Background coverage: A text exchange appears to show a dispute among American diplomats over whether President Trump was seeking a quid pro quo from Ukraine.A second whistle-blower, said to have firsthand knowledge about the president’s dealings with Ukraine, has come forward.