"GDP is an important metric for determining how much the United States could afford to spend on defense, but it provides no insight into how much the United States should spend. Defense planning is a matter of matching limited resources to achieve carefully scrutinized and prioritized objectives. When there are more threats, a nation spends more. When there are fewer threats, it spends less. As threats evolve, funding should evolve along with them. ... Unfortunately, setting defense spending at four percent of GDP would shield the Pentagon from careful scrutiny and curtail a much-needed transparent national debate."
Debate: Four Percent of Gross Domestic Product for Defense?, Center for Arms Control and Non-Proliferation, February 26, 2009