Responsible in what way? Instead of being vague, why not mention specifics?
The US Military does not operate under any religious views and people of all religions are members of the Military and practice their respective religions freely and openly. Osama Bin Laden operated under a purely Religious point of view. The US worked alongside, though not directly with, Osama and his Mujahideen against the Soviets back in the 80's when they were trying to take over Afghanistan. Even then, Bin Laden didn't like the US. However when Saudi Arabia accepted the US help when Saddam invaded (rather than accepting Bin Laden's help) he became even more pissed off with us. Basically he felt that no Non-Muslim should be allowed in that area. Eventually he even went as far as to decree that it was "every muslims daily duty" to kill any North American (not just US) with the ultimate goal being regaining Mecca soley and completely for the Muslim community.
He was waging a Holy War in a very literal sense. It's not all that different from the ideology behind The Crusades, it's just the type of fighting is different. That and the existence of almost instantaneous worldwide media brings all the ugly and unfortunate aspects of war into every home.