[anewhope]

It’s not clear the force size you suggest would result in a stable equilibrium. You’re missing one key justification: the ability to dissuade future military competition. A defense force the size of Spain’s would face substantially more threats given the value of US targets and interests, so we’d then need to increase spending. It’s hard to predict what that world would look like. In the past it wasn’t great. Outspending everybody else from the get-go works.