On Friday, April 27, 1973, a dozen armed FBI agents left their headquarters in the Old Post Office building — today a gilded Trump hotel — and marched up Pennsylvania Avenue. Waving their badges, they walked into the White House.

Their orders were to stand guard in the West Wing, wherein lay evidence of high crimes. “They’re going to lock down and secure the business offices, including the president’s,” the special agent in charge of the FBI’s Washington Field Office, Jack McDermott, told an astonished Secret Service man.

Caught in the web of Watergate, President Richard M. Nixon returned from Camp David and found a skinny young FBI man standing at attention down the hall from the Oval Office. Screaming in rage, he grabbed the agent by the lapels. “What the hell is this?” he shouted.

It was the rule of law challenging the power of the commander in chief—and the beginning of the end for Nixon. He knew that he was doomed. Within a year, the president would be named as an unindicted co-conspirator in an iron-clad criminal case. His impeachment inevitable, he resigned the presidency in the summer of 1974.

We now stand on the verge of the same kind of confrontation. ...