And at one time a lot of people truly believed that World War I had solved the problem of large-scale international wars.
Hitler rose, in large part, because people were so terrified of repeating the horrors of World War I that they were willing to do just about anything to prevent World War II.
There's a slightly different mind set today, but it's analogous -- a fear to act until it's almost too late.
You're also making the mistake of seeing radical Islam advancing only in terms of how it perpetuates violence against other Muslims. There's no saying that that's the only message that will resonate from a radical.
A charismatic figure proposing a unifying message if Islamic hegemony could be a very daunting thing to face.