Tony Blair finally brought his long farewell to a close overnight. In office more than ten years, he is easily the most successful Labour leader ever, as well as the ninth-longest serving British prime minister and, with one exception, the longest serving for more than a century.

The exception of course was Margaret Thatcher (just over eleven and a half years in office), and Blair’s big achievement was to make Thatcherism the bipartisan consensus of domestic policy. He dragged his own party, often kicking and screaming, into the modern era, and for that (in my view) he deserves much credit.

But that and the rest of his historical record risk being overshadowed by one thing: Iraq. Blair was the one world leader who had a chance of preventing the Iraq war — the only one whose opposition might have given George W Bush pause. All the rest, John Howard included, were also-rans, but Blair’s support really counted.

New prime minister Gordon Brown is unlikely to change much on the domestic front, partly because so much of the existing policy is already his. But despite his occasional denials, it stands to reason that distancing himself from Blair’s Iraq policy will be one of his top priorities.

In the fascinating, yet-to-be-written story about how Brown became unchallenged heir, the salient point will probably be the desperate desire of many in the Labour Party for someone who was identifiably not-Blair — and that movement in turn was driven primarily by anger and frustration over Iraq. A plan for British withdrawal will be a symbolic break with Blairism as well as a response to a policy nightmare.

In the meantime, Blair has accepted an appointment as envoy to the Middle East on behalf of the “Quartet” of peace mediators (Russia, the US, the EU and the United Nations).

His prospects for success in the role are dim, due to both its inherent difficulty and the fact that Blair’s recent record makes him deeply suspect from the Palestinian point of view. Realistically, however, no-one who was less supportive of Israel would be likely to win American backing.

Blair has in the past at least been clear about the importance of the Palestinian question as the key to peace in the region. Last year he told the Iraq Study Group in the US that “the way to stop the radicalisation of moderate Muslim opinion is to have a positive strategy of resolving that issue”, a position later endorsed in the Baker report but ignored by the Bush administration.