U.S. national defense spending has ranged widely, from less than 1 percent of GDP in 1929 up to 43 percent in 1944. These extremes illustrate that resource allocation to defense can increase rapidly when a war demands it. Focusing just on the post-World War II period, U.S. national defense spending as a percent of GDP has ranged from a high of 15 percent in 1952 (during the Korean War) to a low of 3.7 percent in 2000 (the period of relative tranquility preceding the terrorist attacks of the following year).

Trends in U.S. Military Spending - Council on Foreign Relations Thursday, October 10, 2013 @ 4:54pm