In a warren of low-ceilinged rooms on the ground floor of the West Wing, down the stairs from the Oval Office and next to the Situation Room, Donald Trump's lawyers are waging war.

They're locked in battle with Robert Mueller, the special counsel investigating Russia's interference in the 2016 election, who has indicted 19 people over the past 13 months, five of whom have pleaded guilty. Now he is homing in on the investigation's most powerful subject: the President, whom Mueller wants to testify under oath about what he knows.

It's a dangerous moment for Trump. If he agrees to talk, the notoriously undisciplined President risks making a false statement, which could be a crime like the one that led to Bill Clinton's impeachment. But if he refuses, Mueller could issue a subpoena, instigating a long, high-profile court battle over whether Trump could be forced to testify.