Take a good look at military history for the United States in the past one hundred years.
For the most part the United States minded it it's own business, and the and the WWI hit catching us unprepared. We quickly ramped up, geared up, were woefully unprepared when we got there, and never had much influence on anything there. Our military came home and that was that. WWII was pretty much the same - We were unprepared, late for battle, and again completely unprepared. And again our military came home and that was that. Then there was Korea, and it was the same shit all over again. After Korea we decided not to downsize our military. If our military was needed, from then on forward it would be able to respond in any theater almost instantly.
And for a long time, US military policy was based on oil. If China or Russia cut off the oil supply from the Middle East, everything in the United States would come grinding to a halt - including our military. However, these days the United States is much less dependant on oil from the Middle East.
The problem is the United States has interests in all regions, and a desire to project it's power in all areas of the world. If you are okay with Israel being attacked and the United States not being able to help for two months and not being able to project a large amount of influence in the region for two years, then yes, we can downsize our military.
However, the US military has benefited the world. The first computers were for the military, the Internet itself was by the US military, GPS was originally military (Called TRANSIT, used by the US Navy in 1960)....
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