Winning American support for those talks was a key reason that President Volodymyr Zelensky of Ukraine — who had called ending the conflict his top priority — sought a White House meeting with President Trump. But Mr. Trump withheld $391 million in military aid and asked for investigations that could help discredit his political rivals.

Now three of our Washington-based reporters have the inside story of how and why Mr. Trump pursued the aid freeze over the objections of his top national security advisers.

Another angle: Russia said on Friday that it had deployed a hypersonic weapon capable of easily evading American missile defense systems. That may be part of an effort to pressure Mr. Trump to renew the last remaining arms-control treaty between the two countries, an agreement that limits strategic nuclear missile launchers and deployed warheads and that expires in early 2021.