This is a big week in Westminster. Kofi Annan, former Secretary-General of the UN, visited Parliament to make a big set-piece speech about

This is a big week in Westminster. Kofi Annan, former Secretary-General of the UN, visited Parliament to make a big set-piece speech about the abolition of slavery two centuries ago. Martin McGuinness and Ian Paisley shook hands over power sharing in Northern Ireland. And Tony Blair is expected to announce officially that he's standing down.It's a great demonstration of how political power ebbs and flows. Kofi Annan is down. Martin McGuinness and Ian Paisley are up. And Tony Blair has discovered that, even though he's still in Number Ten, his power to get things done has been leaking away ever since he first announced he'd stand down sometime this Parliament.It's not as if he didn't win a decent majority at the last election. Or that he suddenly stopped being a brilliantly persuasive orator. It's just that everybody knew he was going to go soon. If his successor might undo anything or everything as soon as they take over, why bother doing what Tony wants in the meantime?For a man who's been Prime Minister for 10 years, and who wants to go out on a high note, it must be intensely frustrating. But it just goes to prove the old saying that politicians and sportspeople rarely get to choose how or when they retire. As both Michael Portillo and David Beckham found out, it's horribly humiliating to be sacked in public. It doesn't matter if you're an international statesman or a local councillor, a premier league footballer or a local league player. Once you've gone there's no way back. That's why it's very hard, and very rare, to quit when you're ahead.Which is why, when you consider the alternatives, perhaps Blair has managed his exit less badly than most.