Actually I think it is complete bullshit. I have seen that argument countless times. The person putting it forward never actually has a solution of any type of course.

Radical Islam was subdued by the Crusades, why is today any different? Hell back then, the Muslims were far more advanced than the West, and they still lost.

You have to understand that their goal is world domination. Zawahiri's from Spain to Iraq reference should solidify that for anyone that doesn't get it.