In fact, the retreat had begun in 1947 with the end of British colonial rule in India and continued through the 1950s as former Asian and African colonies sought independence, and the failure, in 1956, of the Anglo-French military expedition to seize control of the Suez Canal was a watershed. But while Mr. Macmillan spoke of the “wind of change” blowing through the British Empire, the nation’s political elites struggled to adjust to any lesser role for Britain in the world. Public opinion pointed in the same direction. Britain, voters were told, had won the war, and heads of state from all around the world had flocked to London for the coronation of Queen Elizabeth II in 1953. Why should it sign up to a European enterprise of defeated nations?

The 21st century’s Brexiteers were every bit as insouciant in their rhetorical disregard of Britain’s relative decline. Nearly four years after the vote to leave, Mr. Johnson, by then prime minister, chose the Royal Naval College in Greenwich, once a hub of the British Empire’s maritime power, to mark the conclusion of negotiations on the terms of Brexit. That 2020 speech, “Unleashing Britain’s Potential,” sought to again conjure an earlier age of swashbuckling adventurism. His secretary of state for trade, Liz Truss, he said, had her teams in place to strike new global trade deals. “This is the moment for us to think of our past and go up a gear again,” he said. “To recapture the spirit of those seafaring ancestors immortalized above us whose exploits brought not just riches but something even more important than that — and that was a global perspective.” Britain was on the threshold of a new golden age.

It was, of course, a fantasy. Mr. Johnson got Brexit through, but as the Conservative pro-European Michael Heseltine has often put it, this is the sovereignty of the man in the desert. The economy has stalled and trade has shrunk. Britain is poorer than it might have been. Its gross domestic product is at least 4 percent — but could be as much as 8 percent — lower, according to independent calculations, while business investment is more than 10 percent lower. It added new frictions to the lives of Britons: new border checks when traveling to E.U. countries, stricter residency rules for living in Europe, fewer opportunities for students to study abroad. Even just using a cellphone while “roaming” often costs more than it used to.

There have been other costs, one of them a weakening of the glue between the nations of the United Kingdom itself. The referendum result was more a statement of English than of British nationalism — majorities in Scotland and Northern Ireland voted to remain. Forced to leave, Scottish nationalists claimed stronger cause to promote their case for full independence from England, and the complex political arrangements for Northern Ireland needed to protect the Good Friday peace agreement between Irish nationalists and British unionists in the province have weakened the cause of the unionists.

Rather than a newly independent Britain cutting a swath on the international stage, economic realities forced cuts in spending on foreign aid and diplomacy. The hopes among Brexiteers for a new Anglosphere, adding the English-speaking Commonwealth nations of Canada, Australia and New Zealand to Britain’s “special relationship” with the United States, turned to dust, and Britain’s privileged place in Washington was lost to Mr. Trump’s disdain for traditional alliances.