The United States has been building up a military presence around Iran for weeks, even as negotiators from both countries plan to meet later this week in hopes of finding a diplomatic solution to the escalating tensions. David E. Sanger, a national security correspondent for The New York Times, explains what President Trump hopes to achieve through potential military action, and why he has chosen this moment. Guest: David E. Sanger, the White House and National Security Correspondent for The New York Times. Background reading: Mr. Trump has said he is considering a...