I reject the premise that any nation in the history of humanity ever used reason to determine its defense policy or military budget. (Not that you actually made that assertion, but let's be clear what you're asking for: a fantasy standard that has no hope of ever being applied by the crazies who run the world.)
Absolutely. I'm asking for a 'what's right' answer that has nothing to do with what's practically possible to implement.
Your post has part of the point - not "any nation in the history of humanity has ever used reason to determine its defense policy or military budget" you said.
Now find me any president who has admitted what you just said as fact. Discussing the issue has as part of it bringing out things like that that aren't the normal discourse.
In theory and under the law, every nation on Earth has protection from every UN signatory waging aggressive war on them. That has a *practical* effect on the use of militaries. It may not be an ideal - but discussing what's right can lead to things like that UN charter that moves us closer to it.
You did not answer the question - what level of safety should every nation have?
The point there is to get you to consider in part, if you want a low standard for a nation, are you ok with it for you? If you want high for you, are you ok with it for them?
Or are you locked into inequality where you want unlimited safety for yourself and unlimited risk and inequality for others? It's good to admit that for once, if so.