In the nick of time, a well-placed source (a senior official who is broadly neutral towards the British government in this fight) has given me his reading of what happened, and where it all turned sour for Mr Cameron. It rings true to me.
Mr Cameron had two problems, as my source sees it. The first was the nature of his demand, and how it was made. In essence, the British did not ask for an "emergency brake" clause or opt-out for financial regulation.
What they asked for was a protocol imposing decision-making by unanimity on a number of areas of regulation currently decided by majority voting. (If you want to be really technical, the choice is voting by unanimity or the special Qualified Majority Voting (QMV) used in the EU, which is a sort of super-majority system taking into account a certain number of countries and also their populations).
As my source puts it, this amounted to a big winding-back of the clock for many EU leaders, setting a "horrendous precedent" that could unravel the single market. As they see it, common rules for the common market have been adopted (with few exceptions, such as tax) by QMV ever since the Single European Act approved by Margaret Thatcher in 1986.
The much-discussed Financial Transactions Tax issue already requires unanimity and therefore could never be imposed on the City of London without Britain's agreement. What is more, as was pointed out in Brussels with some vehemence, when it comes to financial services there have hardly ever been any cases of Britain being outvoted in the adoption of such legislation.
In simple terms, that means that Britain's request to move to unanimity was taken as a huge ask that had nothing to do with the subject at hand (saving the euro) or was a sign of bad faith (because it is driven by mistrust regarding future legislation). In my source's view, Britain also tabled its request very late in the day, simply sending a whole draft protocol to the European Council legal service the day before the meeting without talking the ideas through with key allies and national capitals.
Then, says my source, came the second crunch moment for Mr Cameron. Once the 27 EU leaders gathered in the summit room, a sense rapidly emerged that a lot of countries did not share Germany's enthusiasm for full-blown treaty change, a process which is slow and fraught with risks (the markets might not wait that long, Ireland might have to hold a referendum, Britain would have to get a vote through the House of Commons and so on).
By a certain point on Thursday night, I am told, a majority of countries were growing interested in a quick and dirty legal fix, suggested by the president of the European Council, Herman Van Rompuy. The fix was dreamed up by lawyers working for Mr Van Rompuy. They said that a legal device, known as "Protocol 12", would allow the 27 leaders of the EU to agree most of the new rules and mechanisms for fiscal union in the euro zone by a simple, unanimous decision among themselves.
Suddenly, Germany looked isolated. Mr Van Rompuy, a former Belgian prime minister elected by EU leaders to chair their summits, decided to see if he could sweeten the deal for the wavering EU leaders, and asked Mr Cameron if he would consider dropping some of his requests. This made sense to some leaders in the room. Mr Cameron's demands were already more than many of his colleagues would tolerate, and Britain had already said publicly it would tailor its demands to the scale of the treaty change on the table. Mr Cameron said he would not lower his ambitions, and that his demands would be the same in the event of Protocol 12 being used, or a full-blown EU treaty.
A hostile view of this is that Mr Cameron overplayed his hand. In this version of events, the British prime minister thought the mood of the room was running towards Protocol 12, and because Protocol 12 is decided by unanimity, he thought he had the whip hand.
Instead, my source tells me, the room turned on Mr Cameron. This, I am told, "was the point at which the Protocol 12 route, which requires unanimity, was effectively closed down and one country after another accepted a new treaty at 17+."
Did Mr Cameron miscalculate? Did he want to end up with a treaty being crafted at almost 26, with Britain on the outside? My source is certain that was not Mr Cameron's goal, and my source is not alone in this thinking.
It is now almost inevitable that separate structures be set up, with Britain on the outside, it seems. Talk in London of preventing the "Eurozone-plus" from using the Court of Justice is also a mistake, I am told. Article 273 of the treaty allows just this.
You can choose to believe this account or not. It comes from a single source, who is well-placed but clearly viewing this from a particular perspective.
Time will tell. But what a mess.