When we killed Abu Musab Zarqawi, among his documents was a letter from someone fairly high up in al-Qaeda, identified only as Atiyah. His letter made it clear that the what al-Qaeda wants is the longest involvement possible of Americans in Iraq. Abu Aardvark quotes from the letter, and summarizes it thus:

Al-Qaeda wants American troops in Iraq, not an American withdrawal. It wants a protracted war which allows it to drain American blood and treasure while producing an endless stream of the images of jihadi heroism and American brutality on which its narrative thrives . . .
bloodshed without Americans would do nothing for their global strategy.