From the very conception of the United States isolationism was the theory that was practiced in the U.S. Early politicians and Americans felt that in order for America to get up on her feet she would need to worry only about herself. This policy was practiced for many years. As usual, Americans became greedy and saw a bigger role for the U.S. Because of this bigger role, America through out the idea of isloationism and went out to conquer the world.
The first step in this conquer the world theory was to have an Open Trade Agreement with China. This was accomplished by the Treaty of Kanagwa. The next step was to acquire land. The U.S. first bought Alaska, then we went and took over Hawaii. From there, the U.S. went to some Spanish held territory. By fighting and winning the Spanish American WAr, the U.S. was able to acquire Puerto Rico, Guam, and the Phillipines.
With the ending of the Spanish American War the U.S. was truely able to think of itself as an imperialistic nation.
After the War was over, the U.S. retreated back into its shell. Yet, once agian, the U.S. was forced to come out. World War I was the catalyst that brought the U.S. out of its shell. This war truely closed the book on U.S. isolationism and showed the strength and vitality of the U.S. From World WAr I unitl today, the U.S. has been able to say that it is a true superpower.