Anthony Blinken said "If you're not at the table, you're on the menu.". The US sure lives by the law of the jungle in international relations. Although this has been the case for centuries, this style of foreign policy really got going with WWII. Our country's war materiel production was behind what was necessary at the time to participate in a 2-front global war. Soldiers were training with cardboard weapons, but because we hadn't outsourced our production offshore, we created an economy based on war that was so lucrative for business that that economy has lasted to this day. Such is it that a war economy itself can conquer a nation. Eisenhower warned about this in his farewell address.

There's protecting a country from invasion, and then there's basing a country's whole economy on a continuation of arms sales. The latter provides a perverse incentive to destabilize regions in order to maintain demand for the American arms industry. In the case of defense of the US against invasion, are you honestly suggesting that the country with the highest private gun ownership rate in the world has that to fear in any scenario? Even if we did need a military at all, one that could appropriately be called a department of "defense" would be a tenth of its current size.

Si vis pacem, para bellum (If you want peace, prepare for war)

If you aren't ready for war, you get conquered by a war monger. If you want to be at peace, you must be ready for war against those who want to conquer.