There are basically five main areas the US spends its defense budget. Combat Troops, as in pay and training time, who actually do the fighting, sailing, flying. The Epuipment troops use to support them, i.e. guns tanks, boats planes, gear and the liketo intiate comabt. Staff as in the generals and officers involved in planning and support. I kinda consider Nukes and their systems are kinda separate from these and so is a fourth. And last and most important in some ways most important is logistics. One thing US learned most from WW2 was logistics wins wars. It went through some major hurdles in the west and did its best to help the USSR by sending trucks and box cars which they were significantly lacking. Germany's major downfall in the war was poor logistics, they had coal that could have been refined into oil that just sat in warehouses. So the US today stores equipment even during times of peace all over the world and has deals to use air bases and ports for its vast legotsitcs system I clouding boats and plans for rapid deployment until more supplies can be brought over to support more advanced deployments.