Zmatrix: Yup, many aren't thinking very clearly. The US and it's Allies need to clean up their own backyards first. These terrorists didn't just arrive in the US early Tuesday then hijack their connecting flights to LA. Many of them have been in the US for a few years! There are likely to be hundreds more waiting for their time to carry out their mission.
I'm going to quote from memory a famous Canadian military historian Gwen Dyer: "At the height of Britains military power, the Afghans defeated Britain. At the height of the Soviet Unions power, the Afghans defeated the Soviet Union. That's not to say that they can't be defeated, but you need a point of operation such as Pakistan and be willing to stick it out for many years. It can be done, but it's going to be awfull bloody. Those hills swallow up armies!"
Things I would concerned about:
1) If China makes a move against Taiwan, will the US have anything left to defend Taiwan?
2) When US casualties begin piling up, will the American public begin to waver in their support.
3) Related to point 2. In 2 to 3 years of fighting will the American public's support begin to wane?
4) How are other Middle Eastern nations going to react? Will they cut off oil supplies to the US?
5) Can the economy withstand a long very expensive war, especially, as mentioned in point 4, if other Middle Eastern nations cut off oil to the US?
6) Will China or some other nearby nation throw their support behind Afghan if the US overcommits it's forces? Remember that China doesn't have much of a Navy or Airforce, but their nearby proximity and the sheer power of their vast Armies could make them an overwhelming force.
Make no mistake, this will not be a walk in the park! This could easily end up being another Vietnam or, on the extreme end, be the prelude to Armaggedon.