"What nation is invulnerable to terrorism?" A lot of them.
My guess is that all of 'em except the US are when they are told about it beforehand.
And according to FBI personnel, the US had enough credible information to expect planes to be hijacked and crashed into buildings.
Except that Condaleeza Rice, the NSA, said, "Who would have thought they'd do THAT?"
So, my guess is that pretty much all countries can prevent it when they are told. As was the US.
Arf