A common trope of hawkish foreign policy writers is that America took a "holiday from history" by starting too few wars and trimming military spending in the 1990s. The unsubtle suggestion is that this holiday caused 9/11. But a better analogy would be that America, for decades, has taken a holiday from arithmetic, spending money like Jill Kelley at a JSOC mixer. 

America is so comparatively wealthy that it has spent roughly what the entire rest of the world spent on defense for 20 years. Even this figure is somewhat misleading, since if you include the spending of our allies and partners across the world—whom one presumes we're not preparing to fight wars against—we currently spend between two-thirds and three-fourths of world military spending.