Terrorism is a tactic. The thing it is especially useful for is facing a better armed opponent. When the opponent is greater in force then the only options you have are to die, surrender, or strike back in some other means. And if you have any sense of self worth the first two options suck.
So the terrorists choose to strike back by other means. And the thing of this is that the opponents military is rendered moot. Because they do not choose to engage the military. Instead they go after infrastructure, economic targets, and morale. The things that enable the conflict. It is the conflict and not just the military that is the threat to people. And they will strike at whatever threatens them in whatever way they deem necessary to survive as a people.
So here the US is using its military might to press its presence on the rest of the world. No longer able to lead with the power of ideas it needs to have a military funded by more money than the rest of the world combined. And suddenly it meats a group who completely end run our military to strike at us. It kind of makes all that money seem ill spent. It makes the idea of organize conflict seem stupid. As if there were a polite and proper way to kill other people. Our proper military activities have lead to more death of civilians than any of the terrorist activities. There is little difference to the people being killed between being killed by terrorists vs proper US military intervention.
As St Augustine observed when an Alexander the Great caught a pirate. The dialog went thus:
Alexander the Great: What meanest thou by keeping hostile possession of the sea?
Pirate: What meanest thou by seizing the whole earth; because I do it with a petty ship, I'm called a robber, whilst thou who dost the same with a great fleet art styled emperor.
For fun here is an animated take on this story
http://www.piratesandemperors.com/ .
The real threat of terrorism is it undermines the power base of those trying to extend their reach to the entire world. It is a means by which those with little or no power can strike back against the interests of those with power.