It was the spring of 2018 and President Donald J. Trump, faced with an accelerating inquiry into his campaign’s ties to Russia, was furious that the Justice Department was reluctant to strike back at those he saw as his enemies.In an Oval Office meeting, Mr. Trump told startled aides that if Attorney General Jeff Sessions would not order the department to go after Hillary Clinton and James B. Comey, the former F.B.I. director, Mr. Trump would prosecute them himself.Recognizing the extraordinary dangers of a president seeking not just to weaponize the criminal justice system for political ends but trying as...