Twenty-five years ago I was thrown out of a party. Tony Blair had been elected leader of the Labour Party on 21 July 1994, and a victory party was held at Church House in Westminster. Journalists were not invited, but I was writing a book about him, so I went anyway.

I listened to Blair’s speech. “A particular thank you to a friend of mine called Bobby, who some of you will know. He played a great part and did so well.” Most people in the room had no idea “Bobby” was Peter Mandelson’s codename among the inner circle, a necessary subterfuge because many other Blair supporters were so deeply suspicious of him.

That is the thing about politics: even at momentous turning points in history, the “who hates whom” is often as important as the great questions of national destiny. It will be the same in the circles around Boris Johnson this weekend.

Anyway, I was politely asked to leave by Tim Allan, one of Blair’s advisers, but I was glad to have been there – on the day the Labour Party stepped decisively into the unknown, electing a leader who struck fear into the hearts of the Conservatives because he was stealing their best tunes.

For half of the time since then – the following 13 years – Blair dominated British politics almost as much as Margaret Thatcher had done before him. Yet his very success generated a double backlash that means politics today is the opposite of what he and his supporters hoped for on that sunny afternoon in the precinct of Westminster Abbey.

The first backlash was against his decision to join the invasion of Iraq. This was a cause of the Labour Party’s turn to the anti-American, traditional socialism of Jeremy Corbyn – even though that occurred 13 years later, and eight years after Blair stepped down.

Just this week, Blair accepted – more than he had done before – that there were arguments against military action to which he gave insufficient weight at the time. In an interview with Steve Bloomfield for Prospect magazine, he still insists Iraq was “not a bad or ignoble mission”, but says that “there were elements that were missing” from the five tests for military intervention he set out in Chicago at the time of the Kosovo war in 1999.

One is whether “there are going to be strong Islamist influences at play”, as there were in Afghanistan and Iraq, he says, and as others have found in Syria and Yemen. The other is whether a long-term commitment is viable if “your own country is divided about it”. This test of public opinion is one Blair dismissed at the time, imagining that, if the intervention were successful, the public would come round to it.

The second backlash against the Blair years was on the European question. For all his time as Labour leader, he used Europe skilfully to divide the Tories. As Philip Collins, his speechwriter, noted in The Times this week, Blair was asked about a referendum on the EU in his final prime minister’s questions. Blair commented on the visceral growl of support from the Tory benches: “If I were the leader of the Conservative Party I think I should be worried about that.”

But it turned out to be not just David Cameron who should have been worried. The whole pro-European cause was set back by the long, slow-burning reaction to a decision taken by the Blair government.

For all that Blair is criticised by pro-Europeans for failing to show leadership on the European question, he took one decision, almost casually, that had momentous consequences. When 10 new countries joined the EU in 2004, he decided to allow their citizens to exercise the right of free movement to come to live and work in the UK from day one.

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This principled decision of European solidarity and liberal economics contrasted with the approach of Germany and France, which was to impose temporary restrictions on free movement for the first few years. But it was a decision made by default, according to Ed Balls, who was then a Treasury adviser: “We didn’t see the extent to which low-wage people would move. Fundamentally, we didn’t think they would.”

By 2016, there were two million more EU-born people in the UK. Their hard work contributed to economic growth, but their arrival also transformed the debate about EU membership.

If Blair had made a different decision in 2004, it is easy to imagine that the referendum might have had a different outcome.

As it is, the Britain of 25 years ago seems an altogether different country, one inspired by the hope of moderate social democracy and by the possibility of a constructive role for the UK in Europe.