Why the Trump administration delayed nearly $400 millions of dollars in security aid to Ukraine is the question at the heart of the impeachment inquiry into President Trump.
Democrats say the president tried to coerce an ally to help him take down a political opponent. Republicans argue it's a routine use of presidential power.
Interviews with current and former officials show how the Trump administration's hold-up of aid to Ukraine was irregular and likely violated U.S. law, and has far-reaching consequences at home and overseas.