WASHINGTON — The setting was the annual dinner of the American Enterprise Institute, a tony affair on Thursday featuring 1,500 exuberant believers in free enterprise, limited government and the superiority of American values.

The question for Boris Johnson — former mayor of London, former British foreign secretary and current potential British prime minister — was simple:

What’s the worst mistake you’ve ever made?

There were many possibilities to choose from. But Mr. Johnson looked at his interlocutor, Arthur C. Brooks, the institute’s president, and developed the glint in his eye that usually means he is about to deploy a well-rehearsed bluster-and-deflect response.

“My strategy is to litter my career with so many decoy mistakes, nobody knows which one to attack,” Mr. Johnson declared. “In the last few minutes I’ve probably said something that the British media will say is absolutely outrageous, though I don’t know what it is.”

What Mr. Johnson did not mention was the cloud of intrigue, both personal (he is about to get a divorce) and political (he is probably plotting against Prime Minister Theresa May), wafting around him as he made his way across the Atlantic.