Two lessons from Johnson’s downfall are paramount. First, tell the truth, even if it makes you look bad, temporarily. The 1971 release of the Pentagon Papers demonstrated that Johnson lied continually when he lauded the progress the United States and its South Vietnamese allies were supposedly making. By 1966, the press was accusing the administration of creating a credibility gap that only yawned wider as the conflict escalated.

All presidents lie at times, but those who admit mistakes, particularly obvious ones, can retain their popularity. This happened to John F. Kennedy after the Bay of Pigs fiasco in 1961 and to Bill Clinton when he acknowledged his affair with Monica Lewinsky in 1998 (although he benefited more from the Republicans’ failed attempt to throw him out of office).

Mr. Biden made a decent start at such truth-telling during his speech this week. But he should give a fuller explanation of why his administration failed to prepare for a Taliban victory that, according to years of intelligence reports, was quite likely.

Second, keep the coalition that elected you united in its response to the crisis.