For outsiders, Brexit has the feel of a long-running soap opera: a mash-up of plot twists and tragic characters loaded with entertainment value but not much significance if you’re not British.

That complacency is a mistake. Never in the last 70 years has a major advanced economy left a free-trade area. Brexit is providing the first real-world evidence of the costs that come from undoing the intricate bonds of globalization.

It is...