Freedom from terrorism, I think.
Obviously, there were a lot of factors that led to 9/11, but that was one of the first major attacks on mainland America that really got the public's attention.
America isn't used to that. They're rarely targeted in their own back-yard. Sure, plenty of American forces have been involved in wars, but it's usually America going to them, rather than them coming back at them.
To them, the war is fighting for their freedom from terrorist groups. I can see where they are coming from, as obviously terrorists attack civilians based on the belief that they are just as responsible for American military action worldwide as the soliders themselves.
They want the freedom to be able to go to work, be out in the city etc without some nutter flying a plane into a building.