1) PROCESS OF ELIMINATION:
There was a tremendous power vacuum left behind after World War II. Pre-WWII, the world's powers arguable included France, Britain, Germany, Japan, and the United States. World War II devastated France. It was defeated by Germany, and for a while ceased being a sovereign nation. Its economy and military was ravaged by the Nazi occupation and after the war was still desperately trying to put itself back together.. Germany was ultimately defeated and occupied by the Allies and was seperated into two pieces. Japan also suffered much the same way as Germany for fighting on the Axis side. Britain was weakened by a combination of both world wars. The strain of two massive global conflicts eventually stretched the British Empire to its breaking point.. The USA (and Russia) were the only major nations still left relatively standing. They would proceed to compete with eachother for superpower status.
After WW2
n March 1941, the U.S. began giving direct support to the British in the form of arms and ammunition through the Lend-Lease Act. After Pearl Harbor, in December, America would become directly involved in aiding the British in Europe. In January 1942, Roosevelt and Churchill agreed to establish a Combined Chiefs of Staff and to the make defeating Germany their first priority. (Winning the war in Europe would come before winning the war in the Pacific.)
http://www.worldwariihistory.info/in/Britain.html
3) MILITARY MIGHT:
The United States mobilization during World War II was a tremendous feat. General George C. Marshall turned the US military (then 17th in size) into a fearsome fighting force which was present on almost every battlefield in almost every theatre during the World War. By 1945, there were around 13 000 000 men and women in uniform. This jump-started the United States armed forces' tradition of excellence which continues today.
4) INDUSTRIAL MIGHT:
The US wartime economy was a force to be reckoned. By 1945, they...