Piling on to a growing body of research, including my own, here is Andrew Cockburn:

By 2008, according to a U.S. Strategic Command study, our military was simultaneously engaged in no fewer than 285 human network attack programs.

So, now that assassination is an official tool of U.S. foreign policy, along with trade embargoes and overseas aid, it is surely time for an open debate on whether it is indeed effective. Surprisingly for some, evidence based on hard numbers demonstrates unequivocally that the answer is no.

More here.