Laura Rozen notes intelligence officials questioning whether the recently-killed Abu Azzam was really Zarqawi's second-in-command for al-Qaeda in Iraq after all. On the other hand, Bill Roggio says that whoever he is, Abu Azzam was still important, and also puts up a handy flow chart noting that several top al-Qaeda operatives have been captured of late. The Belmont Club says that decimating the upper ranks of al-Qaeda like so really does have an effect:

But the worst of it is the wastage to cadres. Those who write that body counts are a meaningless metric to apply against the insurgency ignore the fact that formations which sustain heavy casualties lose their organizational memory while those who suffer lightly retain them. Lt. Col. Joseph L'Etoile is on his third and half of his men are on their second tours of Iraq. For Abu Nasir and many of his foreign fighters, the memory of what to avoid next time has been lost on this, their last tour of Iraq.

Well, in some ways that's true. Note that the Phoenix Program during the Vietnam war, a CIA assassination campaign intended to find and kill Viet Cong cadres in the south which was similarly measured in body counts—and, for that matter, ended up killing lots and lots of South Vietnamese civilians—really did ended up weakening the Viet Cong infrastructure in the south. Overall, the program was a massive failure, and alienated much of the rural population, but it proved that, strictly speaking, if you kill enough people, you can destroy an organization, that the ranks aren't infinitely replenishable. On the other hand, nothing like the Phoenix Program is going on in Iraq, and Douglas Farah's analogy seems far more apt:

Having covered conflicts and the war on drugs for two decades now, it is clear how unhelpful it is to repeatedly trumpet the supposed damage to an organization when one person is taken out of action. The closest parallel I find is in the drug wars, when first Pablo Escobar then other leaders of the Medellin cocaine cartel were taken down. Then the leaders of the Cali cartel were killed would simply step into the breach. While each generation of traffickers or arrested, then the Northern Valley gangs were decapitated. At every step, the DEA and U.S. government would hail the actions as a major triumph, destined to end or greatly diminish drug trafficking. Yet, after each major killing or arrest the amount of cocaine entering the United States remained unchanged. New people was able to individually control less of the market, and each succeeding organization was small and less vertical in its structure, the aggregate amount of drugs they are able to produce and export did not diminish, and ultimately grew.

There's no reason to think Zarqawi doesn't operate like that, especially since everything we've seen has indicated that attacks continue even after this or that latest "top lieutenant" has been captured or killed. Meanwhile, as Anthony Cordesman argues—and U.S. military officers in Iraq are now recognizing, according to the Washington Post—Zarqawi and the foreign fighters have essentially "hijacked" the Sunni insurgency, and are steering it less in an anti-occupation direction, although there's that, and more in a pro-civil war direction, by directing an increasing number of attacks against Shiites and other Iraqis.

What about the rest of the insurgency? The Baathist and Iraqi "nationalist" elements, according to the Post, seem now to have quieted down—content to lay low for now, infiltrate the new government, and are perhaps waiting to stage a coup a few years down the road. Who knows? Nevertheless, Cordesman has also noted before that Zarqawi receives ample domestic support from the thousands of radicalized Iraqi salafists who grew up during Saddam Hussein's "revival of Islam" campaign in the Sunni provinces during the 1990s. (Ahmed Hashim's old analysis of the insurgency here still holds up incredibly well.) Cordesman points out that "rifts" between elements of the insurgency are few and far between, even if some Sunni clerics have been denouncing Zarqawi. This may be because, as Anthony Shadid recounted in his recent book, those clerics discredited themselves among the young Iraqi fundamentalists by their collaboration with Saddam's regime, much as the Shiite clergy in Najaf partly discredited itself among the young Sadrists.

What this all means, it's hard to say. It doesn't seem like the most active elements of the Sunni insurgency, currently, would lose steam if the United States announced a pullout right now. The obvious way forward, which seems to be the U.S. military's current strategy, is to focus mainly on uprooting and weakening Zarqawi's network—at least to the point where it can be handled by a native Iraqi force—which would drastically reduce the risk of Sunni-Shiite civil war, ala 1980s Lebanon, after the United States starts drawing down. Weakening Zarqawi would also, as Army Maj. Gen. Richard Zahner says, allow the political process to "mature." At least that's the hope. If everything written above is correct, then it's not an unreasonable strategy, I think, but it's also not clear that the U.S. can actually do it, as per Farah's post, or that Iraq would stay intact even if you killed every last member of Zarqawi's network (along with all the civilians standing in the way). There are still a hundred other sources of major instability, including the new constitution, or the squabbles in Kirkuk, or the skirmishing among Shiite radical militias, or the hundreds of thousands of ex-Baathists now biding their time.