When historians look back on the events of the past few weeks, what will be the event they point to as the defining “moment” after which it became inevitable there would be a vote of confidence in the leadership of Theresa May?

When Margaret Thatcher faced just such a vote nearly 30 years ago—even though she had won three general elections to become the longest-serving prime minister of the 20th century—historians pointed to the moment when Geoffrey Howe, her hitherto loyal deputy prime minister, stood up in the House of Commons. Once mocked by Denis Healey, the Labour politician who said that an attack by him was “like being savaged by a dead sheep”, Howe delivered the knock-out blow to Mrs Thatcher’s chances of remaining as prime minister.

Speaking sotto voce, he said that Mrs Thatcher had undermined her cabinet ministers at every turn: “It is rather like sending your opening batsmen to the crease only for them to find, the moment the first balls are bowled, that their bats have been broken before the game by the team captain.” In a direct appeal to fellow MPs, he concluded: “The time has come for others to consider their own response to the tragic conflict of loyalties with which I myself have wrestled for perhaps too long.” In the leadership contest that followed, Mrs Thatcher narrowly missed winning the required 15% majority, and after insisting that she would “fight on and fight to win”, thought better of it and resigned.

Today, almost certainly, it will once again be a speech in the House of Commons that historians will see as the turning point. But which one?

Not Jacob Rees-Mogg’s. Last month, on the day that Mrs May presented her EU Withdrawal Agreement to the House, he stood up and, with a cavalier flourish, asked the prime minister if, “as what my right hon. Friend says, and what my right hon. Friend does, no longer match”, he should “write to my hon. Friend the Member for Altrincham and Sale West”: Sir Graham Brady, the chairman of the 1922 Committee that oversees party leadership elections.

Her answer was unconvincing, and Mr Rees-Mogg promptly sent a letter to Sir Graham, calling for a vote of confidence. But his followers didn’t follow, the requisite target of 48 letters needed to trigger a vote was not then reached, and the promised Brexiteer cavalry charge was stopped in its tracks. Any leadership hopes Mr Rees-Mogg may have harboured were dashed there and then.

So how about Jeremy Corbyn’s speech after Mrs May announced on Monday that she would postpone the “meaningful vote” on her EU Withdrawal Agreement? Well, not his either. When he spoke, it was the same old same old. “The government has lost control of events and is in complete disarray,” he said. He concluded: “If she doesn’t take on board the fundamental changes required, she must make way for those who can.” But he lacked the impassioned, inspirational words required to reflect the historical importance of the moment.

So if the speeches of Mr Rees-Mogg or Mr Corbyn do not match Mr Howe’s in significance, then which one does? The best candidate is John Bercow’s.

At just before 4pm on Monday, Mr Bercow, the Speaker of the House, stood up to deliver what turned out to be a withering rebuke of Mrs May’s government. Normally heard bellowing “Order! Order!”, Mr Bercow spoke softly, in a way that seemed almost deliberately reminiscent of Geoffrey Howe. In excruciatingly polite language that recalled the genteel cricketing metaphor favoured by Mr Howe, he tore into Mrs May’s last-minute decision to postpone the vote. “Halting the debate after no fewer than 164 colleagues have taken the trouble to contribute,” he opined, “will be thought by many members of this House to be deeply discourteous.”

No-one was under any illusion as to the Speaker’s true meaning, despite the careful parliamentary language—not the politicians, and especially not the investors. Indeed, by the time Mr Bercow sat down, the pound had plummeted to its lowest point in nearly two years. A first-class student of politics, who studied with the great politics professor and psephologist Anthony King, he must have known the likely impact of his words.

They were devastating. And, with the benefit of a couple of days’ hindsight, we can now see that the die was cast.

By the following day, the government launched an ad hominem attack on Mr Bercow, who voted “Remain” during the EU Referendum in 2016. Andrea Leadsom, the Leader of the House, was despatched to question his motives for intervening in the way that he did. But this was a low, unwarranted blow. He was merely upholding the age-old tradition of the Speaker of the House to remain ruthlessly impartial—a tradition that goes back to another time when the country was bitterly divided: the civil war of the 1640s.

In January 1642, on the eve of war, King Charles I went to parliament to arrest five members of the House of Commons. When he arrived, and saw that they had escaped, he said: “I see the birds have flown”. Turning to the Speaker, William Lenthall, whom he had appointed, Charles demanded to know their whereabouts. Lenthall refused to divulge any information, saying: “May it please Your Majesty, I have neither eyes to see, nor tongue to speak in this place, but as the House is pleased to direct me, whose servant I am here.”

That was a pivotal moment. If Charles didn’t realise he was in for a fight beforehand, he knew then. Likewise, if Mrs May didn’t realise she was in for a fight before Mr Bercow’s speech, she knew afterwards.

It captured the moment—and it will be recorded in the history books as a turning point.