One is the antithesis of the other. The only think they have in common is antipathy towards the West.
Sometimes that's enough.
Let's assume you're correct.
Why wouldn't Saddam find it advantageous to direct Islamic fundamentalists away from his own regime and towards the U.S.?
Furthermore, Saddam has funded Islamic fundamentalist terrorism against Israel. Why wouldn't he also do so against the US, depsite they alleged differences?
