Before the World War II, the economic boom, which had followed the period of reconstruction after the First World War. However, in mid-1920s, the workers movements were failed. That came to an abrupt end with the Wall Street crash and the Great Depression. Unemployment speared from the US to all the country in the world. People do not have enough money to buy things, so that everyday thousands of food wasted and throw into the sea.
At the end of World War II, all the allied countries all went bankrupt except for America. They could not pay for the debts. Europe and the rest of the world need to spend a long time to rebuild the economy. However, all of Europe was devastated and incapable of feeding itself let alone reconstruction. US dollar stays at the stable level, but other currencies were all flexible. At that time, America already built a thriving manufacturing industry, and selling the weapon to other countries. Although US owned a lot of gold, more manufacturing capacity and more military power than rest of the world, but the US could not live without the markets and allies. US needed to rebuild the capitalism, especially in Europe.