Jill Lepore sizes it up:
The United States spends more on defense than all the other nations of the world combined. Between 1998 and 2011, military spending doubled, reaching more than seven hundred billion dollars a year—more, in adjusted dollars, than at any time since the Allies were fighting the Axis.
David Silbey, who posts the above chart showing defense spending as a percentage of GDP, puts the absolute numbers in perspective:
American defense spending dropped dramatically post-1945 (there was a peace dividend), bounced up for Korea and Vietnam (though never to WWII levels) and then trickled down to 2001, rising only marginally for the wars in Iraq and Afghanistan. We simply haven’t spent that much–measured against the size of our economy–on our 21st century martial adventures. In fact, what this looks like is nothing more than an imperial power tending to its empire. Neither the Romans or the British, at the heights of their empire, spent that much on it, instead relying on a relatively inexpensive imperial constabulary to do it. That allowed them empire on the cheap, something that we are managing now as well.
(Chart from usgovernmentspending.com)