“Them’s the breaks,” Prime Minister Johnson acknowledged in front of 10 Downing Street as the one-time mayor of London and an irreverent force in English politics announced his resignation as head of Britain’s Conservative Party. The BBC reports that he will endeavor to stay on as prime minister until a new Tory leader is chosen.  

The departure of what the Sun has called the tow-headed titan throws into doubt the cause of Brexit, the issue on which  Mr. Johnson rose to power. “The herd has moved,” Mr. Johnson observed of the political currents that forced him to abdicate what he called “the best job in the world,” albeit part of a “Darwinian system” that eventually proved too treacherous for even Mr. Johnson.   

The next act in British politics is already being written. In a speech delivered this week, Sir Keir Starmer, Mr. Johnson’s primary political foe and Labor’s leader, promised to “make Brexit work.” He added, “Let me be very clear: With Labor, Britain will not go back into the EU. We will not be joining the single market. We will not be joining a customs union.” That promise now appears set for testing in the wake of Conservative disarray.

While the proximate cause of Mr. Johnson’s resignation is a sequence of scandals over sex, money, and Covid that marked his premiership, it comes at a moment when important elements of Brexit are yet to be redeemed. It opens the way for a new era in English politics, one that could feature a return to power of a Labor Party that suffered a massive defeat but three years ago. 

The British Broadcasting Corporation reports that Mr. Johnson hopes to stay on as a caretaker prime minister until a Tory successor is chosen this autumn. That timeline has already drawn fire from those within the Conservative Party demanding an immediate exit. 

All this marks a stunning reversal that will be studied for generations. Only two and a half years ago, in December 2019, Mr. Johnson secured an electoral victory that was the Conservatives’ largest since Margaret Thatcher’s in 1987. He did it  on a promise to “Get Brexit done.” In recent months, he has emerged as one of President Zelensky’s key allies in Ukraine’s fight against Russia. Now, his departure leaves Mr. Starmer, among others, with a shot at the top job.   

Mr. Johnson’s simmering crises, which began with a slow drip of disclosures, came to a boil amid a rash of resignations. They began with the Chancellor of the Exchequer and the Health Secretary, then matured into a full-blown exodus, with more than 50 secretaries, ministers, trade envoys, and political aides beating a path for the exits.  

Just as damaging was the news that the secretary for leveling up, Michael Gove, who once blocked Mr. Johnson’s chance at 10 Downing Street but eventually became part of his government, called for Mr. Johnson to quit as prime minister posthaste. In that demand, he was joined by the man Mr. Johnson just hours before appointed as Chancellor of the Exchequer, Nadhim Zahawi.  

In an effort to stanch the bleeding, Mr. Johnson axed Mr. Gove, and took to the House of Commons to battle for his political life, explaining that the “job of the prime minister in difficult circumstances, when he’s been given a colossal mandate, is to keep going and that’s what I’m going to do.’’

On Thursday, the reality dawned that he could no longer keep calm and carry on. A no-confidence vote last month, which Mr. Johnson won by a tally of 211 to 148, offered a slight reprieve but masked an erosion of  support within his own party.

The prime minister was brought to this precarious position by the lingering political fallout of “partygate,” a series of disclosures that social events at 10 Downing had broken the Covid lockdown rules with which the rest of the country was forced to abide. 

Mr. Johnson was fined for attending his own surprise birthday party. He offered a “wholehearted” apology, and following a police investigation emerged as the first PM found to have broken the law while in office.

The ostensible cause of Mr. Johnson’s downfall was another scandal, this one centered on deputy whip of the Conservative Party, Chris Pincher. Brought in to government by Mr. Johnson to advise on the no confidence vote, a night out that featured allegations of excessive drinking and repeated groping prompted an apology and resignation. He said “I’ve embarrassed myself.”

The situation grew more serious for Mr. Johnson’s political fortunes when it emerged that the PM had known of previous groping allegations in 2019, and hired Mr. Pincher anway. For many, this was an error in judgment and the subsequent obfuscation was the last straw. 

The scramble to succeed Mr. Johnson as head of the Conservative Party will likely begin immediately, despite Mr. Johnson’s hopes for a protracted departure. Contenders include one-time foreign secretary Jeremy Hunt, deputy prime minister, Dominic Raab, the recently resigned Chancellor of the Exchequer, Rishi Sunak, army veteran Tom Tugendhat, and foreign secretary Elizabeth Truss.

Words that Mr. Johnson penned in 2010 might soon redound on him: “As I write these words, Gordon Brown is still holed up in Downing Street. He is like some illegal settler in Sinai, lashing himself to the radiator.” He asked “Isn’t there someone —  the Queen’s Private Secretary, the nice policeman on the door of No 10 — whose job it is to tell him the game is up?”