I understand the grievances of many of those who voted to leave the E.U. For starters, they felt swamped by E.U. immigrants. (The E.U. should have protected the U.K. from that surge; that was German and French foolishness.) There are reportedly some 300,000 French citizens living in London, which would make it one of the biggest French cities in the world. I had a drink with a member of Parliament in the bar in the House of Commons on Tuesday, and as we sat down he whispered to me that “not a single person working in this whole building is British.”

I also get the resentment of Brits at having regulations set by faceless E.U. bureaucrats in Brussels. And I get their resentment at the globalized urban elites, who those in the rural areas here believed looked down at them. And I get the squeeze on middle-class wages here that gets blamed, unfairly, on the E.U. and immigrants the way President Trump blames Mexicans. I get all of that.

But I also get what it means to be a leader in the 21st century. And it sure doesn’t mean asserting your sovereignty over all other considerations or breaking out of the giant E.U. market, where the U.K. sends over 40 percent of its exports, without a serious national discussion of the costs and benefits.