I wrote about this a few days ago when he ducked Katie Couric’s question by torturing the distinction between tactics and strategy. According to The One, the president sets the strategy: Most troops out in 16 months but some left behind for various missions. The generals supply the tactics: To carry out those missions responsibly, we need X number of troops. What does X equal? Why, it’s … “entirely conditions-based”:

In Iraq, it’s not new that Prime Minister Nuri al-Maliki has wanted to take control of his own country. But there’s always been this gap between his assessment of his abilities and American commanders’ saying he’s not up to it. As president, faced with that difference between what he says he can do and what the commanders say he can do, how would you choose between them?

Iraq is a sovereign country. Not just according to me, but according to George Bush and John McCain. So ultimately our presence there is at their invitation, and their policy decisions have to be taken into account. I also think that Maliki recognizes that they’re going to need our help for some time to come, as our commanders insist, but that the help is of the sort that is consistent with the kind of phased withdrawal that I have promoted. We’re going to have to provide them with logistical support, intelligence support. We’re going to have to have a very capable counterterrorism strike force. We’re going to have to continue to train their Army and police to make them more effective.

You’ve been talking about those limited missions for a long time. Having gone there and talked to both diplomatic and military folks, do you have a clearer idea of how big a force you’d need to leave behind to fulfill all those functions?

I do think that’s entirely conditions-based. It’s hard to anticipate where we may be six months from now, or a year from now, or a year and a half from now.

Team McCain points to Bob Novak’s column this week citing unnamed Obama advisors as saying this could mean leaving as many as 50,000 troops in place. According to a recent essay by Colin Kahl, who runs Obama’s working group on Iraq, in the “near term” they might keep as many as 12 brigades there for “overwatch,” i.e. support, duties.

If Obama’s top priority really is withdrawal, his Iraq policy should begin by setting the number of troops he’s comfortable leaving in the field and then asking for recommendations on which missions are feasible given that number. The fact that he’s going about it the other way, starting with the missions and then building any drawdown around them, is a decidedly McCain-esque (i.e. conditions-based, i.e. responsible) approach. He tweaked McCain this morning for having lately come around to so many of his own positions, but in light of this, he and Maverick are almost mirror images on Iraq now: McCain thinks troop levels should depend on conditions but concedes that 16 months is a “pretty good timetable” whereas Obama thinks 16 months is a pretty good timetable but concedes that, er, troop levels should depend on conditions. Nuance. Predictably, the McCain camp is crowing about it. Here’s their statement, hot off the presses:

“Today Barack Obama finally abandoned his dangerous insistence on an unconditional withdrawal of U.S. combat troops from Iraq by making clear that for the foreseeable future, troop levels in Iraq will be ‘entirely conditions based.’ We welcome this latest shift in Senator Obama’s position, but it is obvious that it was only a lack of experience and judgment that kept him from arriving at this position sooner.

“John McCain has always held the position that any withdrawal from Iraq must be based on conditions on the ground. With the incredible success of the surge, which John McCain advocated, it is increasingly likely that U.S. troops will be able to withdraw with victory in hand. John McCain had long urged Barack Obama, who opposed the surge, to return to Iraq in order to see the immense changes in the security situation there since his last visit. Now that Obama has finally met with General Petraeus, it appears that he has also come to the conclusion that troop levels in Iraq must be based on the conditions on the ground.”

The key remaining conceptual difference between them is, of course, the type of missions they have in mind for residual troops. McCain surely imagines something more ambitious, Obama something more limited and support-oriented. Watch for the debate to shift to that subject next, especially in light of the big AP story this afternoon talking about troops in the field already shifting to peacekeeper roles (which they’ve had for awhile in some parts of Iraq) and reconstruction support. Are U.S. peacekeepers out of the question for President Obama? We’ll see.

Update: Per the last paragraph and the evolving scope of the mission, a reader notes that Obama’s residual force would theoretically contain no combat troops. Big difference with McCain, to be sure, but again — read the AP story. There’s not much combat going on in Iraq anymore that would require combat troops anyway. The issue now is peacekeepers, troops who are going to walk the beat, see sporadic action, and reassure Iraqis that there’s a strong security presence available to deal with contingencies while the IA gets up to speed. How about it, BO?