After months of anticipation, British Prime Minister Tony Blair has announced that he will leave office June 27. Mr. Blair, who had kept his country on tenterhooks since revealing in 2004 that he would not complete his third term, leaves a mixed legacy. He transformed the Labour Party, which had been dominated by stalwart leftists and seemed relegated to perennial opposition. Unfortunately, that accomplishment will be twinned with his support for U.S. President George W. Bush and the war in Iraq, a policy to which he remains deeply committed, despite growing differences with the rest of the country.

Mr. Blair is one of the world's diplomatic senior statesmen, even though he is just 54 years old. He has been prime minister for over a decade and has outlasted virtually all other heads of state. Well trained by the rough and tumble of the British political system, he is a thoughtful and eloquent spokesperson, although critics — on the right and the left — charge that eloquence as mere glibness that masks a readiness to put political expedience above conviction. Such criticism is inevitable when a politician tries to modernize his or her party, an effort that invariably alienates the old guard within the party as well as rivals across the aisle.

And Mr. Blair was nothing if not successful. While it is common today to speak of fatigue or a Labour Party in decline, it would be irresponsible to forget that Labour was viewed as irrelevant 15 years ago; some dared to suggest that it would be eclipsed as the major opposition party in Britain by the Liberal Democrats. In the early 1990s, the Conservatives dominated British politics, enjoying the legacy of "the Iron Lady," Ms. Margaret Thatcher, a divisive and domineering prime minister who was the longest-serving office holder of the 20th century. Labour had been marginalized and the British left watched with envy as U.S. President Bill Clinton overhauled his Democratic party and triangulated his way to office.