We out-spend the next ten largest defense budgets in the world, and that of those only China at #2 spending a little over 1/3 of what we spend, and Russia at #3 spending under 1/10 are enemies. The rest of the next 10 are either formally allied with us, have common interests is there's a big war (India), or are clients of ours (Ukraine). Given that, the problem is not how much we spend, but what we spend it on. Something needs to be done to abolish "inter-service rivalry" which costs us by making Congress give something to all the services, even if the only real need is (take your pick, fleet modernization or some improvement to ground forces), and the pork-barrel aspects of defense spending that tries to get contracts in as many Congressional districts as possible (cf. the joke about the "invicible weapon system: it has a contractor in every Congressional district).
We should stop building hangar-queens for the Air Force (I'm thinking of the F-22), streamline defense procurement, and buy only what we actually need to meet current and reasonably foreseeable threats without trying to give each service a cut of the pie. If serious cost-benefit analyses were applied to defense procurement without respecting inter-service rivalries, or where the contractors are located, just looking at the benefits to our defense posture, not whether it will make, say, the Navy, unhappy, or doesn't have a contractor in the district of the House Appropriations Committee's district, we could cut defense spending without harming defense.
