After WWII many countries had been ravaged by the fighting and much of their infrastructures destroyed. Britain was bankrupted by the conflict and had to negotiate for it's colonies to be come independent as it didn't have any money to continue it's empire. The world needed rebuilding and was payed for with American dollars. America had actually profited and grown economically during the war and was now in a position to dominate financially with the dollar becoming the reserve currency of the world. This meant the dollar was where other nations chose to invest their wealth. The dollar was the safest investment to make. What's more international trading in oil and gold could only be conducted using U.S. dollars. Basically, countries needed (and still do) dollars to pay their bills. In 1971 President Nixon took the dollar off of the 'Gold Standard' so that the U.S. could increase the supply and by default the dollar became linked to the most valuable and sought after commodity - Oil.