Let’s widen the aperture a bit. This contention was a subject of debate in the White House on Wednesday, according to the Wall Street Journal:

Mr. Obama, in a cabinet-level meeting on Wednesday, pressed his military commanders over whether the Taliban still has close ties to al Qaeda and whether the international terrorist group would continue to have a haven should the Taliban regain control of parts of the country, according to a senior administration official.

"There are people over there that think that there’s a rift between the Taliban and al Qaeda," said a senior military official. "The logic is that since the Taliban once owned Afghanistan, and got kicked out of Afghanistan, they’re not likely to make the same mistake twice."

But the official added that "it’s a small number of people who think that," and that most officials involved in the debate are convinced that the Taliban "will at least be complicit in allowing al Qaeda back in" if they regain control.

As I wrote last week, it’s a great question and ought to be thoroughly debated. But does it go far enough? Let’s say the Taliban comes back into power in Afghanistan and, for one reason or another, al-Qaeda doesn’t follow Mullah Omar across the border. Would that be sufficient for the protection of U.S. interests in Afghanistan against al-Qaeda? Or would there be additional considerations: the loss of an Afghan staging ground to harass or contain al-Qaeda in Pakistan; the prospect of money, material and resources trafficking across the border to the Pakistani tribal areas; and other elements that would constitute Afghanistan providing not safehaven for al-Qaeda, strictly speaking, but strategic depth? 

I’m not saying I know this would happen. I don’t know that it would. Much depends on the circumstances under which the Taliban returns to power. (A power-sharing deal; an element of reconciliation; outright victory; etc.) These are questions that need to be studied, not off-the-shelf answers just out of reach of the debate. There are good questions being asked of what "safe haven" really entails, and to the pot let’s add the concept of strategic depth.

Update: An excellent follow-on observation from Matthew Yglesias:

You don’t need Kabul to be able to provide a safe haven. Nor do you need the majority of Afghanistan. Afghanistan’s almost as big as Texas; you could presumably fit a safe haven into a rather small fraction of the country.

Yes, absolutely, it’s not a binary. That sharpens the point considerably, and begs the question of the degree to which, under the above framework, al-Qaeda has that strategic depth right now. If so, that means the clear-and-hold counterinsurgency effort really is directly related to the counterterrorism goal all sides in the debate endorse. If not, it calls into question the entire purpose of the enterprise.