Defining moments don’t usually occur this early in a presidential season, but an exchange between two Democrats, Sens. Hillary Clinton and Barack Obama, seems to have produced one.

The issue was how to manage relations with hostile foreign powers – specifically whether either would commit to meeting personally, without prior conditions, with the leaders of Iran, North Korea, Venezuela, Syria and Cuba. It was a test of judgment and how each candidate understood the functioning of the presidency.

Obama made the pledge without hesitation, arguing he would end the Bush administration’s policy of never talking to the bad guys. George Bush thinks it confers a certain legitimacy on rogue leaders just to be in his presence. Clinton demurred at once. No such commitment should be made, she said, without first exploring what’s at stake and making certain the meeting will not make the president an unwitting instrument of foreign propaganda.

Who came off better? Their difference dominated political discussions all last week, with the consensus being that Clinton had carried the day. As the Economist, the London-based magazine, observed, it’s “probably unfair to imply that Mr. Obama wants to skip off for a kaffe klatch with Hugo Chavez (Venezuela’s boss) right away. But it did make him look a bit naive. The point went to Mrs. Clinton … .”

That analysis, however, overlooks the reality that Clinton and Obama, for political reasons, were addressing two electorates – Clinton a general election audience from which she’ll need the support of independent voters to win the White House, Obama an almost exclusively liberal Democratic audience that will choose his party’s nominee.

Their strategies are understandable. Clinton enjoys a comfortable lead in the polls among Democrats and believes she’s got the nomination locked. She doesn’t want to screw up her general election chances by appearing to pander to the Democratic left. Obama’s got a different problem. He trails Hillary and must shake her standing among Democrats to have any chance for the nomination.

If the audience and postdebate reaction are any measure, Obama won the exchange. But for anyone who understands how foreign policy is made and the risks involved in meetings between world leaders, Clinton gets the prize.

Few things do more damage to relations between nations than a failed meeting between their leaders. A good example is the too-quick meeting between John F. Kennedy and Russian boss Nikita Khrushchev at the height of the Cold War. Kennedy was anxious to sharpen his foreign policy credentials. The preliminary spadework was never adequately done by aides. The meeting was a disaster.

They agreed on nothing, they exchanged bitter accusations, the freeze between Washington and Moscow deepened, and military spending in both countries skyrocketed. The Cuban missile crisis, which brought both countries to the nuclear brink, was one result.

In contrast, Wily Dick Nixon’s meeting with Mao Zedong took place only after months of secret negotiations led by Henry Kissinger. Each side knew in advance what the other needed and what it could concede. Nixon and Mao met merely to sign the paperwork; the devilish details had already been hammered out. It’s the prudent way to do these things, as Clinton knows.

The Clinton-Obama exchange on the issue tells us something about how Clinton would function as president that’s reassuring. She’s cautious. It tells us something about an Obama presidency that, at first blush, is unsettling.

It’s conceivable Obama’s still too green to fully understand the intricacies and ambiguities that define foreign policy and the presidency. But I don’t think so. Despite his youthful appearance, Obama, at 45, is no kid and gives every sign of being a quick study who’s unlikely to be as rash as president as he seemed in the debate.

That said, the jury’s still out on Obama. He remains a work in progress. With Clinton, you pretty much know what you’ll get. It’s one of the biggest differences between them.