This, buried in the 9/11 Commission's official report, is a start:

The Sept. 11 Commission has found that the Bush administration's war on terror has severely impaired al Qaida's ability to organize another spectacular attack against the U.S. homeland by capturing or killing the deadly terror group's key leaders, drying up their financial resources and severely limiting their ability to "strategize, plan attacks, and dispatch operatives worldwide."

(Shamelessly stolen from Nick)