Jo Swinson almost got it right. She was bold, taking a risk for the chance of great gain, having decided that, otherwise, the cause was lost. That cause was keeping Britain in the EU, and by the time she decided to go for a general election, she thought we were headed for the exit. She thought an election was the only way to stop it.

That was the bit that the short-serving leader of the Liberal Democrats got wrong. It is easy to say, in hindsight, that her decision to allow an election was a disaster, for the pro-EU cause, for her party and for herself. But the Lib Dem review of the election, published yesterday, confirms that it should have been clear at the time that the gamble was so unlikely to pay off as to be reckless.

The report, a readable, thoughtful and honest account written by Dorothy Thornhill, a Lib Dem peer who was the directly elected mayor of Watford, reminds us of the state of politics in October last year. Boris Johnson had just won a vote in the House of Commons on the principle of his withdrawal agreement. The parliamentary deadlock was breaking. Several Labour MPs finally voted with the government. But it was as if, instead of crossing the Rubicon, the Brexit army waded into the river and set up camp in the middle of it.

Many of those Labour MPs — Lisa Nandy, now shadow foreign secretary, most prominent among them — said they would try to amend the bill as it went through parliament, and that they would vote against it if it failed to offer sufficient protections for workers’ rights and the environment. Swinson judged — correctly, I think — that the Labour resistance to Brexit was broken. The opposition to the withdrawal bill had come down to a question of how many days MPs should spend debating it: it was therefore just a matter of a few weeks before the legislation would pass.

So the choice was to go for broke, or to play a long, defensive game. Thornhill’s report does an excellent job of explaining how Swinson thought she had a chance of stopping Brexit — but that, even if she failed, she thought the party would gain seats and emerge stronger.

In this, she put too much weight on the defections to her party of Chuka Umunna and Luciana Berger, and on opinion polling. The report points out that Swinson ignored the warning signs: the party only just won the Brecon by-election in August, as the Brexit Party vote was squeezed, and in October, after a party-conference bounce, the Lib Dems were already drifting downwards in the polls.

And she miscalculated the power of “get Brexit done” as a message; the chance that the Brexit Party would vanish overnight; and how the unreadiness of the Labour Party would favour Johnson and not her.

It was her decision on 28 October to support an early election in a Commons vote the next day that sealed the nation’s fate, and her own. Everything else was detail. The Lib Dem policy of revoking Article 50, and the hubristic nonsense of her ambition to be prime minister, were minor errors compared with the one big mistake of giving Boris Johnson the election he wanted.

Thornhill’s report criticises the role of “an ‘inner circle’ of advisors at arm’s length from the resources of the party machine”, with “decision-making in the hands of an unaccountable group around the leader”. All of which may be true, but none of it subtracts from Swinson’s responsibility for the big decision.

Sometimes, the bold course is right, and even if the gamble fails, people can see that it was worth taking. Not in this case. The cynical thing to do would have been to continue to deny Johnson an election. He would have had to stumble on without a majority. He would probably have taken the country out of the EU, on 31 January or soon thereafter, but he would have been hobbled by a hung parliament in the trade negotiations that followed.

The Labour Party would have struggled on under Jeremy Corbyn’s leadership — and we can see now more clearly how dysfunctional that was. I am not sure how good that would have been for the country, but it would have been better for the Lib Dems, and for Swinson herself as one of the leaders of the opposition with whom Johnson would be forced to share power. Instead, she is said to be thinking about trying to be elected to the Scottish parliament.