[Zak]

at certain scale, it's about ensuring that nobody even tries to lift a finger against you. And then there's the 'implied offensive' angle where others fear what you might do

All true, but I think these things are true of China, with 1/5 the spending or the UK with 1/10. It is probably possible to provide an effective defense, including the ability to effectively attack an overseas enemy spending a much smaller amount than the US actually does.