Does military spending buy security?

Source: The Economist

According to an article in The Economist, the United States is spending 4.7 percent of GDP currently on the military. In the projection above, U.S. military expenditure is projected out at three percent of GDP. Three issues:

• I think projecting GDP growth out four decades is very dodgy. Think about the things that can intervene in a political economy that change the growth rate.
• It would be a very good idea to find a way for China and the United States to live in peace. A war between two enemies each capable of spending more than a trillion dollars a year would be very bad for the world.
• Reducing U.S. military expenditures would help a lot in bringing federal expenditures into balance with government income, and if the balance were properly invested could contribute significantly to our economic growth.