Throughout history, chaos has often been a crucible of great leadership. Yet with Britain in the throes of its biggest political crisis since World War II, it will be surprising if that turns out to be the case this time.

Boris Johnson prepared to become prime minister on Tuesday after the Conservative Party chose him as its leader. A master of political theater and the quotable quip, he is idolized by rank-and-file Tories determined to quit the European Union no matter the consequences. Yet his record as journalist, legislator, London mayor and foreign secretary displays far more bluster than achievement, and a disdain for hard work, probity or the truth.

The prospect of a Johnson government has already spread alarm even through the senior ranks of his party. Several Conservatives voted with the opposition last week to block possible efforts by Mr. Johnson to circumvent Parliament in his drive for a no-deal Brexit. Philip Hammond, the finance minister, and Rory Stewart, the international development secretary, announced that they would quit the cabinet upon his ascendance. Alan Duncan, the Foreign Office minister, resigned on Monday and David Gauke, the justice minister, said he would resign if the next prime minister pursued a no-deal Brexit.

The chief challenge before Mr. Johnson comes on Oct. 31, when Britain must leave the European Union with or without a deal. Theresa May, who resigned as prime minister after her plan for a negotiated break with the European Union was thrice rejected by Parliament, managed at least to postpone the deadline.