WASHINGTON — As President George Bush prepared to order American troops into war to eject Iraqi invaders from Kuwait, he feared it could end his presidency. “If it drags out,” he dictated to his diary on Dec. 20, 1990, “not only will I take the blame, but I will probably have impeachment proceedings filed against me.”

Eleven days later, in a letter to his children, he quoted a Democratic senator telling him that “if it is drawn out,” he should “be prepared for some in Congress to file impeachment papers.” On the day the war began , a Democratic congressman did just that, introducing a resolution of impeachment accusing him of “conspiring to commit crimes against the peace."

Fortunately for Mr. Bush, the war was relatively brief and efforts to impeach him fizzled. But he was hardly the only president to worry. While President Trump is just the fourth commander in chief in American history to confront a serious threat of impeachment, the prospect hung over many of his predecessors, a nagging worry in the back of the mind for some, a constitutional sword of Damocles for others.

Impeachment has served not just as a means for removing a corrupt president from office, as outlined in the Constitution — in fact, it has never actually accomplished that purpose. Presidents Andrew Johnson and Bill Clinton were both impeached by the House but acquitted after Senate trials, while President Richard M. Nixon resigned before the full House could vote. But impeachment has served as a deterrent, a consequence that presidents had to consider when making decisions that crossed into questionable territory.