WHEN prominent people in Washington spend an anniversary apologizing for being catastrophically, unforgivably wrong about a decade-old decision, you might expect that the decision in question had delivered their party to disaster or defeat. But last week’s many Iraq war mea culpas were rich in irony: one by one, prominent liberals lined up to apologize for supporting a war that’s responsible for liberalism’s current political and cultural ascendance.

History is too contingent to say that had there been no Iraq invasion in 2003, there would be no Democratic majority in 2012. (It’s easy enough to imagine counterfactuals that might have put Hillary Clinton in the Oval Office.) But the Democratic majority that we do have is a majority that the Iraq war created: its energy and strategies, its leadership and policy goals, and even its cultural advantages were forged in the backlash against George W. Bush’s Middle East policies.

All those now-apologetic liberals who supported the war in 2003 are a big part of this story, because without their hawkishness there would have been no antiwar rebellion on the left — no Michael Moore and Howard Dean, no Daily Kos and all its “netroots” imitators.

This rebellion divided the Democrats, but it also energized them. During the long Reagan era, American liberalism was an ossified establishment pitted against a successful right-wing insurgency. But the anti-Iraq war insurgency created something new in modern politics — a kind of “movement liberalism” that thought of itself in the same scrappy, ideologically driven terms as the conservative movement, and that was determined to imitate conservatism’s tactics, institutions and success.