Okay so there are a couple of common phrases that often are used to describe the US president: "leader of the free world", "the most powerful man in the free world (or just the world), "the most powerful office in the free world (or just the world)", and "the most powerful person in the world (or just the world)". The general consensus is that the President started to be considered any of these phrases as the Cold War started. The earliest usage that I can find is in 1952 in this [New York Times article.] (http://select.nytimes.com/gst/abstract.html?res=F10C1FF9355E107A93CBA9178AD95F468585F9) The reason that the President gained this title is that the United States was the first nation to have developed nuclear weapons. The Soviet Union then developed nuclear weapons of their own in 1949 and then the two nations were the two most powerful nations. The "free world" portion comes from the falling of the iron curtain around Europe and the Soviet Union becoming the hegemon of that portion of the world.