What caused the relative decrease in violence in Iraq?

One of the assertions in the new Bob Woodward book is that the decrease in violence in Iraq in 2007 and 2008 was not caused 100%, or necessarily at all, by the troop increase. I know, crazy. Because clearly a decrease in violence that began, oh, five months or so after the troop increase was because of that troop increase, and no other factors should be considered.

Anyway, Woodward spelled out his argument in an article on Monday accompanying the series of book excerpts.

Woodward's case, based on his new information, is that the decrease in violence may have been largely because of U.S. targeting of certain individual group leaders:

Beginning in the late spring of 2007, the U.S. military and intelligence agencies launched a series of top-secret operations that enabled them to locate, target and kill key individuals in groups such as al-Qaeda in Iraq, the Sunni insurgency and renegade Shia militias, or so-called special groups."
...
a number of authoritative sources say the covert activities had a far-reaching effect on the violence and were very possibly the biggest factor in reducing it.

Ok, so that's not the argument I would necessarily believe, but who knows. I don't think assassinations, even in a war context, have all that good a record of leading to peace and security in the longer term.

But I am happy that Woodward is supporting the idea that maybe the decrease in violence wasn't caused by the increase in troops -- even if I'm skeptical of his alternative reasoning.

The idea that the troop increase wasn't the only cause is nothing new; the left has been pushing it, Obama and other Democrats gently suggest that maybe it was just one part of the reason, and Juan Cole has made the case, and I think he probably knows what he's talking about.

I'm optimistic that Woodward, a figure of the mainstream, might help mainstream this idea.