The US has sort of gotten itself into an off situation.

Basically, North America is part of NATO. As is europe.
Under NATO law/ruling, if one of the member states is attacked as an act of war, it will be deemed as an attack on all member states, and there should be a collaborated retaliation.

President Bush declared that the terrorism was an act of war against the US, thus bringing NATO into the situation.

So, because the US is bound by NATO, they cannot launch an attack by themselves. It must be done with NATO.
Obviously there's nothing stopping Bush from attacking anyone, but it wouldnt be a wise move politically speaking.
Reason being that for one problem he's abiding by international treaties, and another problem he's decided to forget about them.