Well, consider the situation of Tony Blair, who has already had a cabinet secretary, Clare Short, threaten to resign on him if he goes to war without further UN authorization. Consider also that she may take other ministers with her, and you see that if Blair goes to war without doing something to improve matters, he runs the risk that his government will disintegrate out from under him. It is difficult to imagine him going to war under those circumstances, so he has to do something to improve things.
Now consider that Friday afternoon, after Dubya's press conference, Ms. Short got a private progress report from Blair, after which she expressed delight at the shiny bauble the world had just received from Britain's special friend, and hinted that depending on how matters went further with Iraq, she might not resign from the government after all.
And so to the Azores, and the meeting which the White House was going out of its way this morning to characterize as anything other than a "war council", lest someone be tempted to resign on principle from a government which had already decided on war. I imagine we'll know what Ms. Short thinks of the result of that soon enough.
But then again, this is all speculation, and not particularly well informed at that. Heck, my first thought on hearing Dubya's announcement was that he'd gotten some last-minute pressure from the Saudis. Not a terribly productive exercise, but it beats lighting your hair on fire. At least I like to think so...