How did it happen? Why did David Cameron apparently rip up his own policy of positive engagement with Europe, as well as 40 years of British foreign policy?
"Parliament," one British official whispered to me in Brussels. In other words, Cameron was not prepared to stand up to Conservative Eurosceptics and force through the Commons a treaty to help save the euro - if necessary, with the votes of Labour and Liberal Democrat MPs. True, it would have been messy. There would have been ministerial resignations as well as a huge backbench revolt.
But Cameron accepted that saving the euro was in Britain's national interest. When it came to the crunch, it seems he put his party's interests first.
I have been watching British prime ministers at international summits since 1987. I saw Margaret Thatcher wield her handbag and John Major and Tony Blair veto the appointment of federalists as president of the European Commission.
Cameron went much further. He has broken Thatcher's law: always keep a seat at the table. She knew that was in the national interest.
The Conservative Party has changed since then, as the revolt by the 81 Tories who voted for a European Union referendum in October showed. Before the Brussels summit, there were even threats of a leadership challenge.
Cameron's problem is that the Tory Europhobes are never satisfied. Although they don't admit it, the hardliners see his veto as the start of a process leading to Britain's EU exit.
In another era, Britain's "special relationship" with the United States would have given it another option. The Obama Administration sees Britain as one player in Europe; the danger now is that it is a small rather than a big one. France and Germany will matter more because they will lead a group of 26.
The forces Cameron has unleashed may well end in a referendum after the next general election.
Although the public might like his posturing now, I am not sure they would vote to withdraw from the EU.
A strong business lobby would remind people that three million jobs would be at stake. At some point, Cameron is going to have to stand up to his party rather than pander to it.
BIG NAMES SPEAK UP
Voices from the Liberal Democrats:
Paddy Ashdown, former Liberal Democrat leader:
"We have tipped 38 years of British foreign policy down the drain in a single night. We have handed the referendum agenda to the Eurosceptics. [John] Major had the courage to stand up to his bastards. It pains me to say it, but [David] Cameron has acted as the leader of the Conservative Party and not the Prime Minister of Britain."
Simon Hughes, Liberal Democrat deputy leader:
"[Eurosceptics] should calm down. There will not be an opportunity for them to pull us further away from Europe. That's off the table."
Vince Cable, Liberal Democrat Business Secretary:
"I am not criticising the Prime Minister personally. Our policy was a collective decision by the Coalition. We finished in a bad place."