Two rival noisy demonstrations besieged parliament on the night of the vote on Theresa May’s Brexit deal on 29 March. When it was defeated by 58 votes, both sides cheered enthusiastically. One of them must be wrong, I thought, as I watched the dancing crowds in Parliament Square. Well, now we know.

The demonstrators who opposed May’s deal because they didn’t want Britain to be trapped in a customs union with the EU got it right. They have finally got what they wanted, even if it will be at the price of separating Northern Ireland, a bit, from the rest of the UK.

The other lot, who had come to wave placards to stop Brexit, got it wrong. Britain will be out of the EU on 1 February. Those MPs who voted against May’s deal in the hope that it would lead to a new referendum in which the people would change their minds about Brexit were mistaken.

We can see clearly now: Labour MPs should have voted for May’s deal. We would have left the EU with the guarantee of staying in its customs union. May said it wouldn’t be needed, and if it was it would be a “temporary” customs union, until a long-term trade deal could be negotiated, but the point about the so-called backstop was that it was a fall-back position unless it was replaced by a deal that would have the same effect.

Labour MPs cannot say they didn’t know. It was pointed out at the time that May’s withdrawal agreement was what the Labour leadership said it wanted: it was in every respect what they said a Labour government would have negotiated. The only differences between Jeremy Corbyn and May were about the non-binding political declaration setting out the government’s aspirations for the long-term trade deal.

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By using those manufactured differences as an excuse to vote against May’s deal, Labour MPs more or less ensured that Boris Johnson would become prime minister and thus paved the way for him to take Britain out of the EU on his terms.

It seems unfair that some Remainers blame Jo Swinson for what they call a hard Brexit in the new year. Her miscalculation was more visible. Her pride and her fall a more vivid drama. Everyone knows that we only had the election because she decided to support Johnson’s early election bill, which he tabled to get round the Fixed-term Parliaments Act requirement for a two-thirds vote in the House of Commons.

The bill needed only a bare majority to pass, and Nicola Sturgeon was ready to ask her MPs to vote for it. She calculated – correctly, as it turned out – that the Scottish National Party would win back a lot of the seats it lost last time. And hardly anyone noticed or cared that she was prepared to sell out the Remainer cause for party political advantage.

Jo Swinson addressed public after stepping down from post as Liberal Democrat leader

But the SNP’s votes were not enough to ensure the bill’s passage. Johnson had expelled 21 Conservative MPs, who decided – also correctly, as it turned out – that they stood no chance of re-election as independents. The swing votes on that bill were Swinson’s 20 Liberal Democrat MPs. Yes, I am old enough to remember when the Lib Dems had twice as many MPs as they have now.

It was Swinson’s decision to vote for the early election that sent the voters out on a cold December day, halved her party’s representation in parliament and sealed her own fate.

I’m not saying it was the right decision, but she was probably right to fear that Brexit was going to happen anyway. Curiously, I think Johnson was probably right, too, to fear that parliament would find ways to delay, complicate and frustrate Brexit. He won the vote on the principle of the EU withdrawal bill, but it was never just a matter of time to get it through – it would have been amended and proceduralised and stalled. Nevertheless, in the end, I think Brexit would have gone through, because Labour MPs in Leave seats were starting to crumble.

Swinson was responsible for delivering a Conservative majority government, but it was Labour MPs who gave us Johnson as prime minister and his Brexit.