Maintaining the United State's huge military machine is not cheap. Every year the U.S. spends hundreds of billions of dollars on the military ($346,500,000,000). Since 1948 the U.S. has spent more than $15 trillion to build up its military might. Just how much is $15,000,000,000,000 worth? It adds up to more than the cumulative monetary value of all human-made wealth in the U.S! In other words, the government has spent more on the military over the last four decades than the value of all the factories, machinery, roads, bridges, water and sewage systems, airports, railroads, power plants, office buildings, shopping centers, schools, hospitals, hotels, houses, etc., in this country put together!!
During the Cold War the U.S. had a serious military competitor in the U.S.S.R. Today the U.S. maintains a huge war machine despite the lack on of any serious competition. The U.S. militar budget is now larger than the next 15 biggest spenders put together. It makes up a full 36% of total global military spending. I guess being the world cop and all, we do have certain responsibilities.
If this shocks you, you should read "Addicted to War" by Joel Andreas.