Based on what we experienced in Vietnam, and our increasingly tenuous fiscal issues here at home, can the United States continue to be the world's policeman?
There is no objective answer to this. There are a number of issues to be considered.
First, there is clearly the financial issue that you allude to in the question. It does cost a great deal to “police” world and we are not in the best of financial circumstances right now. However, there is also a more political issue. When we act as the policeman for the world, we acquire many enemies. People who do not like the way that we are policing can come to hate us. This can lead to things like problems with terrorism.
However, we cannot simply say that we should give up policing the world. You can argue that the world needs to be policed. For example, if pirates were free to roam freely around the Horn of Africa, the world’s supplies of oil could be compromised. If ruthless dictators were allowed to have nuclear weapons, they could create havoc. These sorts of things could be very bad for the world as a whole. In this sense, we have to ask whether we can afford NOT to be the world’s policeman. If we do not police the world, it might become much more chaotic and that could be worse for us financially than spending the money to act as police. Moreover, if we do not police the world, another country might take over that job and do it in a way that we would really hate.
Thus, it is not easy to simply say that we should pull back from our role as the world’s policeman.