You left out some possibilities. Perhaps religion both makes a counttry weak (and thus vulnerable to imperialism) and leads to terrorism? Or perhaps weakness makes a country vulnerable to imperialists, and also vulnerable to religious extremism?

And the other flaw in this reasoning is that there are a whole lot of places that were formerly imperialist subjects or were treated as pawns during the Cold War. They're not all full of anti-Western terrorists now. Pretty much the only ones that are are the Islamic ones.