You opened an intellegent line of discussion
You're going to hate what I have to say about it.
You are absolutely correct that the world needs a police force. However, America taking on this role by herself is more like the neighborhood bodybuilder playing bouncer when he hears about a domestic disturbance. It is *not* like calling the cops. What happens when the bodybuilder thinks one of his neighbor's wives is kinda cute?
Until you have the world getting together and setting some kind of international law (implying the consent of the majority of those governed by that law), and setting up an impartial enforcement regime, then *at best* America stepping in is vigilante justice. At worst, it becomes America beating up on whoever she does not like.