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Defense spending stood at 6.8 percent of GDP at the height of the Reagan defense buildup. But, beginning even before the breakup of the Soviet Union it began a decline, reaching below 6 percent in 1990, below 4 percent in 1996 and bottoming out at 3.5 percent of GDP in 2001, about half the level of 1985.
But 9/11, the terrorist attack on iconic US buildings in 2001, changed that, and defense spending began a substantial increase in two stages. First, it increased to 4.6 percent by 2005 for the invasion of Iraq, and then to 5.0 percent in 2008 for the the “surge” in Iraq.
Spending increased further to 5.7 percent in 2011 with the stepped up effort in Afghanistan. Defense spending is expected to decline to 4.5 percent of GDP in 2015 and 3.8 percent GDP by 2020."
http://www.usgovernmentspending.com/defense_spending