Jane Galt has a question today: Why did we become the world's policeman?
This is a question I have some insights into not already offered in the comments to her post.
After the Second World War the United States was ascendant. Our secure base across the ocean had in no way been devastated by the conflict, unlike Europe, Japan and the Soviet Union. At the time, the United States economy was approximately half of the entire world economy. To repeat: HALF of the ENTIRE WORLD's economy was U.S. This was clearly a temporary situation resulting from the massive destruction caused in Europe and Japan and didn't last, but clearly the U.S. was the most powerful nation in the world.
With this much power, it was virtually impossible for the United States to return to its isolationist roots. This was made all the more evident by the emergence of the Soviet Union as a rival ideological and political force. The Soviet Union was a clear threat to the U.S. directly (through nuclear weapons) and indirectly (by threatening to unite Europe under its iron fist, in which case it would have the productive capability to match the U.S.).
The traditional security guarantor of last resort was the United Kingdom, which attained this role as a result of its expansive colonial conquests and leading role in naval power projection capability since the defeat of Napoleon. As a result of the threat from the Soviet Union and Communism, the United States replaced the UK in this role, starting by replacing the UK's security guarantee to Greece as it was under attack by Communist partisans in 1946. The UK handed the torch of "world's policeman" to the United States from this point on.
We currently guarantee the security of Europe through NATO, Japan through bilateral agreements, Australia and New Zealand through the ANZUS treaty, Southeast Asia through ASEAN, and the Middle East through agreements with Saudi Arabia, Israel, Egypt and Jordan. We ARE the world's policeman, whether we like it or not, and it is a result of historical factors as well as the military weakness of our (sometimes) allies. It can certainly be argued that this weakness is often caused by our role as policemen, and that our role incurs a great deal of resentment. But, above and beyond our not insubstantial humanitarian foreign aid, our contribution to the well-being of everyone on Earth is made by securing trade routes on which much of the world's economy depends and greatly reducing political conflict throughout the globe, even if we don't go around reshaping governments or becoming involved in every catastrophe. That is our role, though it is little acknowledged, the U.S. plays a key role in ensuring that civilization as we know it does not fall apart.