If you want to know why US foreign policy is the way it is, a big part of the reason is that the overwhelming majority of the financial support for people doing policy-and-politics relevant stuff is defense contractors. Then there’s some money to be raised from people with right-wing views about Israel. Beyond that . . . almost nothing.
A commenter adds:
People who make this point about defense spending being a lower percentage of GDP than in the past are tacitly admitting that the defense budget is not about defense but about the defense industry. If the GDP doubles, does it suddenly become twice as expensive to defend the country? Of course not.
Another commenter made the startlingly obvious point that the Department of Defense does little to actually protect our borders. Instead, its job is to train soldiers to go out on peacekeeping missions and nation-building exercises. In fact, we could probably abolish the DOD today, and that would still leave us with the National Guard and Department of Homeland Security. Why do we need a DOD anyway? (One commenter provides an answer: to protect shipping lanes).