I do not feel that there should be a policeman of the world. If we look at the history of the world, this has sort of taken place in the past, to a certain extent, and turned out terrible. The British tried to enforce their values and morals on America. This only led to the American Revolution. This has been repeated a few times throughout history. Although nations may have a few of the same rules, the world will never operate under ALL the same rules, morals or values. Cultures are too different for that. It would never work.
As far as the policeman goes... In a way we have that. But its not necessarily a policeman. The United Nations provides means to help resolve international conflicts and formulate policies on issues affecting all of us. I feel that the world cannot and will never have just on policeman to govern us, but that we will always have the United Nations to help out with that.
However, I do agree with Kelly in saying that a nation with so many resources has an obligation to help other countries in need.