He did the right thing, but took his time over it. For like Lord Palmerston, one of his most distinguished and professional predecessors as Foreign Secretary, Boris Johnson is a risk-taker who possesses a well-hidden streak of prudence.
When faced with major decisions, he does not like to be rushed, and can spot and defy an artificial deadline with the greatest ease.
So he did not, in the manner of Michael Heseltine, walk out of Friday’s Cabinet meeting at Chequers, and down the long drive to the waiting journalists.
Instead an ominous silence fell over the weekend, while he considered his options, as he did before deciding to join the Leave campaign rather than become an unwilling member of Team Cameron.
But once David Davis had done the principled thing, and refused to promote a compromise in which he did not believe, Johnson could not keep us waiting much longer, without himself looking cowardly and unprincipled.
He took the braver course, which was to resign, knowing that the boldest measures are sometimes the safest. If he had remained in the Government, attempting to justify a policy in which he did not really believe, he would have faced growing humiliations and a growing loss of credibility.
Or as Johnson put it in his resignation letter, “the Government now has a song to sing. The trouble is that I have practised the words over the weekend and find that they stick in the throat.”
Johnson is plainly offering a quite different kind of leadership. He accuses May of seeking agreement with the European Union by making pre-emptive concessions, which will simply embolden Brussels to demand more:
“It is as though we are sending our vanguard into battle with the white flags fluttering above them.”
As people at once spotted, that is as devastating a line as Sir Geoffrey Howe’s about Margaret Thatcher in his resignation speech:
“It is rather like sending your opening batsmen to the crease only for them to find, the moment the first balls are bowled, that their bats have been broken before the game by the team captain.”
But while Howe was condemning Thatcher for being too rigid, Johnson condemns May for being spineless, with the result that we are now “headed for the status of colony”.
This is not all about Johnson. It is about the kind of country which Britain is going to become, and in particular whether we dare, on occasion, to assert ourselves.
May has suddenly lost two of the three Brexiteers who were supposed to endorse whatever deal she reached with the European Union.
She has made huge efforts to keep her party together, and she has failed. A substantial part of the Conservative Party has lost all faith in her, and it is hard to find anyone who thinks she should lead the party into the next general election.
There is now a danger she will carry on for a year or two, but in such a weakened state that she is an even less formidable negotiator, millions of people who voted for Brexit feel betrayed, and UKIP or something like it rises from the grave.