The Secretary of State resists easy definition, and always has. He’s a Vietnam combat veteran with a deep skepticism of military solutions. He’s a loyal Republican–a staffer in three GOP administrations–who disagrees with several conservative tenets of the party. (He’s pro-choice and pro-affirmative action, to name two.) He’s an African-American path-breaker widely regarded for accomplishments that have nothing to do with race. He’s a guy from the Bronx who grew up on the subway, but whose adult hobby is tinkering with a quintessential symbol of suburbia: a fleet of Volvos. Most important these days is this: he has been the only dove in what, in response to September 11, has become a very hawkish White House flock.

But there comes a time in every career when choices have to be made–visible, dramatic choices–and Powell’s speech to the United Nations is, for him, such a moment. He has become the most respected man in public life by virtue of his cautious, moderate, sober, winning, bridge-spanning persona. He hasn’t quite been all things to all men (and women), but close to it–a role model and inspiration with few enemies outside the Beltway. That is about to change. Powell has thrown in his lot with the Bush War Party, and that decision will define his public profile from here on. If he has an elective career ahead of him–and there are those who fervently hope so–he now has as much at stake in the outcome of a war as his boss. He can no longer have it both ways.

The last time Powell took part in momentous decisions was as Chairman of the Joint Chiefs under the first President Bush. In his role, arguing steadfastly against a war to push Saddam out of Kuwait, he was still largely a behind-the-scenes figure. His real views were obscured by the one sensational moment he had on TV, the famous briefing at which he explained, pointer in hand, that the American strategy for fighting the Iraqi army was simple enough: “cut it off and kill it.”

This time around, his anti-war views have become much better known, and his reversal of position all that much more dramatic. His turnabout is not exactly a surprise. Powell is nothing if not dutiful, and he has been obligingly playing his role in the dove-hawk debate, waged against Vice President Dick Cheney and Secretary of Defense Donald Rumsfeld. The president has needed Powell to play this role.

But Powell’s conversion has taken him beyond the minimum amount he would have to do now to be a team player. There are several reasons. From the few times I’ve seen him up close, I can say with certainty that he is a man of immense pride, dignity and self-confidence–a man who does not like to be played for a fool. Some of it’s genetic; perhaps some is the result of having come up through the ranks by way of the ROTC parade ground at the City College of New York. But the French stepped on Powell’s internal landmine when they sandbagged him at the U.N. the other week, inviting him for a discussion that turned into a European anti-war diatribe. Powell is a diplomatic man but, at heart, not a diplomat. I’m told he was shocked–and angry.

Second, the Powell/Cheney-Rumsfeld “split” has always been exaggerated. Powell is too much of a team player to put himself permanently on the outs with the Bush Team, once it was ready to reach a consensus. Powell almost certainly knew where Bush would end up–which is, pretty much where he started last summer–but was willing to give diplomacy a chance because that’s what Secretaries of State do and, more important, that’s what President Bush wanted him to do.

Powell, I’m told, doesn’t hold out more than an angstrom of hope that Saddam & Co. will flee. But Powell knows that, for any psy war to succeed, the secretary of state has to be fully on board. There’s one other thing to consider: the “Powell Doctrine.” Derived from his experience in Vietnam, it holds that if we go in, we go in all the way–and with the full backing of the American people. Powell knows that his assent is critical to Bush’s chances of having a unified country behind him if and when a war starts. The Secretary was willing to spend some of his political capital to see that his Boss begins with as full a supply as possible.

That will not endear Powell to the doves in either party. Nor will Powell’s new tough line convince many GOP hardliners that he can be trusted. They know Dick Cheney. Dick Cheney is a friend of theirs. And they know that Colin Powell is no Dick Cheney. But it may be enough to get Powell on the ticket on the off chance Cheney won’t or can’t run again. The conservatives and hawks are thick on the ground at the Republican grass roots and Powell would need their support. If you want to ride the train you have to get on board, and, at the U.N., that is precisely what Colin Powell has decided to do.