Few observers believed that Obama genuinely intended to break new ground with his response—his campaign had never articulated any such policy before, and seemed ill-prepared to defend it on the spot. The Clinton campaign dutifully pressed the attack the next day, calling Obama’s statement “irresponsible and frankly naive.” But then a funny thing happened. Obama’s team did not try to qualify (or, in political parlance, “clarify”) his remark, and no one said he misspoke. Instead, the campaign fought back, with memos to reporters and with a speech by the candidate himself, aimed squarely at the sort of “conventional wisdom” that had, in the words of his then-foreign-policy adviser, Samantha Power, “led us into the worst strategic blunder in the history of U.S. foreign policy.”

It was only mid-summer, ages before the Iowa caucuses in campaign time, so it was a good moment to experiment. And it worked: polling suggested that Americans were largely on board with Obama’s position. Soon, on the stump, he was regularly referring to his willingness to meet with foreign leaders, unlike other top presidential candidates.

This position really was a departure for Obama. Despite his stand against the war in 2002, he had since hewed closely to the party line on foreign affairs. The only substantive thing he had to say about Iraq policy during his famous 2004 convention speech was: “When we send our young men and women into harm’s way, we have a solemn obligation not to fudge the numbers or shade the truth about why they are going; to care for their families while they’re gone; to tend to the soldiers upon their return; and to never, ever go to war without enough troops to win the war, secure the peace, and earn the respect of the world.” This merely echoed the bland competence-and-execution argument of mainstream party thinking. And as Clinton’s campaign has been at pains to point out, Obama’s Senate voting record on Iraq-related issues is nearly identical to hers. Before the YouTube debate, the higher Obama’s political ambitions had reached, the more cautious his foreign policy had become.

If Obama had followed up his win in Iowa with a victory in New Hampshire, as once looked likely, and then wrapped up the nomination in February, the YouTube skirmish might have remained a curiosity, a small outlier in an otherwise conventional foreign-policy agenda. A “bring home the troops” pitch would have created more than enough contrast with John McCain’s foreign policy, and a strong focal point for debate. With the entire electorate as an audience, it would have been in Obama’s interest to minimize their other foreign-policy differences.

But “bring home the troops” offered no differentiation from Hillary Clinton, who had more or less successfully pivoted away from her earlier support of the war. As the campaign stretched on and Clinton sharpened her attacks on Obama’s commander-in-chief credentials, he began to counter by questioning her whole approach to foreign policy—the establishment approach. Today, Obama calls not only for direct negotiations with leaders of rogue states, but also for an American commitment to eventual global nuclear disarmament (in part to reinvigorate nonproliferation efforts); a substantial rebalancing of American military priorities toward Afghanistan (and away from Iraq); a softening of the embargo on Cuba; and a widening of the current, single-minded focus on democracy promotion to include other development goals that might more effectively prevent terrorist recruitment. Many think that there’s little difference between the Democrats on policy grounds. That may once have been true, but over time—and largely in response to Clinton’s barbs—Obama’s foreign-policy approach has evolved into something substantially different from either Clinton’s or McCain’s.