The problem starts with our defense commitments. Those oceans which to some extent protect us also make it very difficult to project power elsewhere.
We have to protect the sea lanes to get troops and supplies anywhere else. That is massively expensive. If the US were "isolationist" (which I am not suggesting), our defense spending could easily be $100 billion a year, we'd defend our general areas of interest and that would be that. Nobody else could mount any realistic conventional threat to our holdings.
And of course when speaking about defending the nation we also should consider the need to defend our financial security as well. But, fundamentally, I have long felt our System is broken beyond any realistic repair. I think it will fail after I'm gone, but that failure could be epic, and the denouement could be disastrous.
We could have a nice shiny military and a broken country in default.