Trumpism has an essence, and that essence is nationalism. It is bigger than President Trump and certain to outlast his tenure in office.

Mr. Trump’s candidacy began as a furious attack on both the Democratic and Republican political establishments, and a vow to do something neither party had done recently—put “America first.” In both respects, his campaign and presidency have been strikingly similar to the nationalist movements in England and Europe, from Brexit to the euroskeptic governments in Poland, Hungary and Italy,...