But now, out of power for almost seven years, he remains defined in many Britons’ eyes by one issue: his steadfast support for the United States and President George W. Bush in the Iraq war, a decision that isolated him from much of his party and continues to haunt his legacy, especially with Iraq now fracturing into sectarian violence.

Mr. Blair has increasingly been defending himself, and he injected himself into the debate again this weekend, posting a lengthy essay on his website on Saturday asserting, among other things, that it is wrong to blame the 2003 invasion of Iraq by the United States, Britain and their allies for the violent Sunni-Shiite divisions now racking the country.

It is “a bizarre reading of the cauldron that is the Middle East today,” Mr. Blair said, “to claim that but for the removal of Saddam, we would not have a crisis. And it is here that if we want the right policy for the future, we have to learn properly the lessons not just of Iraq in 2003 but of the Arab uprisings from 2011 onwards.

“The reality is that the whole of the Middle East and beyond is going through a huge, agonising and protracted transition,” he continued.

“We have to liberate ourselves from the notion that ‘we’ have caused this. We haven’t. We can argue as to whether our policies at points have helped or not; and whether action or inaction is the best policy and there is a lot to be said on both sides. But the fundamental cause of the crisis lies within the region not outside it.”