The role of America as a world power changed greatly between the years of 1914 and 1950.
During the First World War, the U.S. remained in isolation for much of it, not wanting to get involved in what most people considered a “European war.” Public opinion eventually changed as the war began to spill onto the western side of the Atlantic, but American involvement in the war was more or less an afterthought. The armed forces and navy were a shadow of what they would one day become. Once the war was finished, the U.S. retreated back into isolationism, deciding not to involve itself in world political bodies such as the League of Nations. The Great Depression in the 1930’s increased American’s isolationism as foreign trade broke down across the globe.
At the outset of World War II, many Americans felt that the conflict was European in origin and not worth the attention of our armed forces. Although ideologically many people felt that Germany and Japan were our moral enemies, they refused to get involved directly. American factories produced limited amounts of munitions for the Allies, but it wasn’t until the Pearl Harbor attack that the U.S. decided to get involved in the war in the Pacific and by proxy the Atlantic. During this conflict, the U.S. landed expeditionary forces Europe and Italy and helped speared head the western drive into Berlin. In the Pacific, it was American marines that took critical airfields needed to launch the final bombing raids that forced the Japanese surrender.
After the Second World War, the U.S. rebuilt Japan and most of Western Europe almost single handedly. The wartime boom had left the U.S. as the undisputed world power in the west thanks to a huge production boom, large amounts of unspent war bonds and little to no bomb damage. The Soviet Union soon began expanding to try and protect itself from future invasions and soon the U.S. saw itself as the only possible means of stopping communist aggression and protecting developing democracies around the globe. That is why during the expanding Cold War the U.S. became the leading western nation in most of the conflicts and political contests.