It is not a ponzi scheme because there is no centralized entity paying off later investors with earlier ones, but I would still not invest in it. Way better investments out there. Something can be a bad investment without being a ponzi scheme.
Also, regardless of any morality issues, having a military _creates value_ in a way that Bitcoin cannot. The employees of a military are paid, they spend their wages in the country's economy, they must be supplied with equipment which must be manufactured, those manufacturers & soldiers pay taxes, the enhanced security of a nation makes it less risky for investment and business growth etc etc.
The employees of a military are paid... by taxpayers, who are the ones creating value.
Arguing that military exists for the purpose of protecting currency is akin to arguing that cryptographers exists for the purpose of protecting currency. Technically, that is one of the things they do [for cryptocurrencies and banks alike], but it'd be fairly ridiculous to claim that making cryptographers more useful was some sort of negative side effect of currency comparable to Bitcoin's energy use.
If you read some pretty authorized authors about it, let's say Smedley D. Butler ( https://en.wikipedia.org/wiki/War_Is_a_Racket ), then having one and only one mission for the army (self-defense) may be a better way.