It is a measure of English Brexiteers’ political acumen that they were initially oblivious to the volatile Irish question and contemptuous of the Scottish one. Ireland was cynically partitioned to ensure that Protestant settlers outnumber native Catholics in one part of the country. The division provoked decades of violence and consumed thousands of lives. It was partly healed in 1998, when a peace agreement removed the need for security checks along the British-imposed partition line.

The re-imposition of a customs and immigration regime along Britain’s only land border with the European Union was always likely to be resisted with violence. But Brexiteers, awakening late to this ominous possibility, have tried to deny it. A leaked recording revealed Mr. Johnson scorning concerns about the border as “pure millennium bug stuff.”

Politicians and journalists in Ireland are understandably aghast over the aggressive ignorance of English Brexiteers. Businesspeople everywhere are outraged by their cavalier disregard for the economic consequences of new borders. But none of this would surprise anyone who knows of the unconscionable breeziness with which the British ruling class first drew lines through Asia and Africa and then doomed the people living across them to endless suffering.

The malign incompetence of the Brexiteers was precisely prefigured during Britain’s exit from India in 1947, most strikingly in the lack of orderly preparation for it. The British government had announced that India would have independence by June 1948. In the first week of June 1947, however, Mountbatten suddenly proclaimed that the transfer of power would happen on Aug. 15, 1947 — a “ludicrously early date,” as he himself blurted out. In July, a British lawyer named Cyril Radcliffe was entrusted with the task of drawing new boundaries of a country he had never previously visited.

Given only around five weeks to invent the political geography of an India flanked by an eastern and a western wing called Pakistan, Radcliffe failed to visit any villages, communities, rivers or forests along the border he planned to demarcate. Dividing agricultural hinterlands from port cities, and abruptly reducing Hindus, Muslims and Sikhs on either side of the new border to a religious minority, Radcliffe delivered a plan for partition that effectively sentenced millions to death or desolation while bringing him the highest-ranked knighthood.

Up to one million people died, countless women were abducted and raped, and the world’s largest refugee population was created during the population transfers across Radcliffe’s border — an extensive carnage that exceeds all apocalyptic scenarios of Brexit.

In retrospect, Mountbatten had even less reason than Mrs. May to speed up the exit clock — and create insoluble and eternal problems. Just a few months after the botched partition, for instance, India and Pakistan were fighting a war over the disputed territory of Kashmir. None of the concerned parties were pushing for a hasty British exit. As the historian Alex von Tunzelmann points out, “the rush was Mountbatten’s, and his alone.”