There are a zillion ways terrorists could attack America and the West; subs carrying suitcase-nuke terror squads are just one way.
There's also germ and chemical attacks and even ordinary explosives could be used like at the Boston marathon, because ordinary explosives are very easy to get and smuggle around, for example a huge truck bomb in the heart of Washington, London, Paris, Rome, Denver, San Francisco etc.
As i've said before, America and the West could greatly reduce the risk of such attacks by firstly withdrawing our troops from the mid-east, and secondly by stopping supporting Israel with cash and weapons.
This Koran verse gives a clue-
"And do not kill any one whom Allah has forbidden, except for a just cause" (Koran 17.33]
See, they feel they have a "just cause" because our politicians invaded the mid-east and are also supporting Israel.
And like i've also said before, Israel is bristling with nuke missiles and is well able to look after herself, she doesn't need the West..