The United States transitioned from a gold standard currency due to expenditures from the Vietnam War. Old money from the 1950's said silver certificate due to the fact you could trade the bill in for physical silver. Also half dollars, quarters and dimes from 1964 and earlier had a 90% percent silver composite. The fiat system came in 1971 and went into the current Petro-Dollar system. It makes it so that trade of oil is priced in dollars so everyone has to buy dollars to trade. When looks at the Petro-Dollar system it is easy to see why the United States gets into many of the recent conflicts. Iraq has oil, Libya was trying to circumvent its trade of oil away from dollars. Russia brokered a deal with China on the oil it was not selling due to sanctions due to Crimea.