I think the danger is that we will stop short in prosecuting the war. Sept 11th, happened on the American mainland but all the attacks of the past 30 years (and I include the oil boycott as an attack) continued because no one was made to pay. Great powers should be both respected and feared. When the US is again feared, we will be safe. If the "war" ends in a cave in Afghanistan, the rest of our enemies will laugh...and the countdown to WWIII will continue.
This war may well end in a cave, but it won't be a cave in Afghanistan. We plan to root out every single terrorist hiding in every single cave, rabbit warren, hovel, palace, everywhere and anywhere on this earth. BTW, that includes those terrorists, and their suporters, who live amoung us.
This already is World War III.