When Britain voted to exit the European Union in 2016, Donald Trump, then the Republican candidate for president, declared himself “Mr. Brexit.” Trump claimed solidarity with the nationalism, isolationism, and anti-immigrant fervor that drove Brexiteers and incorporated it into the America First platform he pursued after winning the 2016 presidential race. Brits now view their populist experiment as a dismal failure. Champions of Brexit, such as former Prime Minister Boris Johnson, promised that severing political and economic integration with most of the rest of Europe would boost prosperity and self-sufficiency and leave more money for government services. Instead, the UK...