BBC news: Blair will stand down on 27 June (10 May 2007)

"I decided we should stand shoulder to shoulder with our oldest ally, and I did so out of belief," he said of his decision to support America's invasion of Iraq.

...

"But believe one thing if nothing else, I did what I thought was right for our country. And I came into office with high hopes for Britain's future, and, you know, I leave it with even higher hopes for Britain's future."

In conclusion, he said: "Actually I've been lucky and very blessed. And this country is a blessed nation.

"The British are special - the world knows it, in our innermost thoughts we know it. This is the greatest nation on earth."

But if you want to know why he did it, see this comment subthread about Irwin Stelzer in the diary The rancid relationship by RogueTrooper on March 23rd, 2006.

Bush is a symptom, not the disease.