Measured as a share of GDP, U.S. defense spending is down substantially from the figures of 10% or more that often occurred in the 1950s and 1960s, and down from the 6% reached during the Reagan defense build-up of the 1980s. Although defense spending as a share of GDP has nudged up since September 11, 2001, it was 4.7% of GDP in 2011.
Defense spending also doesn't dominate the federal budget as it once did. Back in January 1961, in President Dwight Eisenhower's farewell address, he warned of the dangers of the "military-industrial complex." But at that time, defense spending was still almost 10% of GDP and more than half of total federal spending. Indeed, defense spending was as much as 70% of all federal spending back in the early 1950s (and higher than that at the peak of WWII). But for the last two decades, defense spending has dropped to about 20% of all federal spending.
While U.S. defense spending at relatively low levels, historically speaking, both relative to GDP and relative to total federal spending, it remains high relative to spending by other countries. Here's a table showing that U.S. defense spending is more than 40% of the world total, and that U.S. defense spending comfortably exceeds the sum of defense spending by the next 10 largest spenders.
The U.S. economy is the largest in the world, and it also spends one of the greatest shares of that economy on defense. By my count, only eight countries in the world (for which SIPRI has data) spend a larger share of their GDP on defense than does the U.S.: Saudi Arabia, 11.2%; Chad, 6.2%; Georgia, 5.6%; Iraq, 5.4%; Israel, 6.3%; Jordan, 6.1%; Oman, 9.7%; and UAE, 7.3%.
Of course, spending isn't the only variable that matters in national defense: strategy, diplomacy, ideology, economic ties, even personal cross-border ties can affect the likelihood and extent of instability. Defense spending can be quite good at projecting certain kinds of power, but not especially useful at blocking a biological or nuclear weapon that fits in a panel truck or even a large suitcase. That said, these sorts of numbers cut both directions in the debate over levels of defense spending. Those who favor reductions in defense spending over time might take note of the fact that we haven't been living in Eisenhower's world for some time, and U.S. defense spending has a smaller share of the economy and of federal spending than the historical norm. Those who favor higher defense spending might take note of the fact that the U.S. is far and away the largest defense spending nation now--and that many of the other largest spenders are our allies.
For both sides, I'm always interested not just in hearing argument about "more" or "less," but about what is enough. If you prefer cutting defense, how low would you go before you would say "enough"? If you prefer increasing defense, how high would you go before you would say "enough"? If someone can't explain their answer to that question, I suspect that underneath their show of confident certainty, they don't really know any more about weighing the costs and benefits of military spending than I do.
Thanks for Danlu Hu for putting together the U.S. defense spending figures over time from the Historical Tables of the President's Budget for 2013.