Trump doesn’t understand the question because he doesn’t understand doing things for the betterment of anyone but himself.

Perhaps so, perhaps not. But that doesn’t make the question any less valid.

For most of history, you didn’t ask “what’s in it for me”

Yeah… that’s not really true at all. Peasant and/or commoner soldiers in both ancient and medieval wars expected to be rewarded with loot and, of course, rapine - that’s the whole reason sackings was such a common thing in those days. Any king or emperor who didn’t provide that was gambling with his own life.

The story of Mulan you mentioned has more to do with Confucian morality than reality - wars in China, by and large, worked on the same rules as those everywhere else. Medieval Japan is a good example - those samurai expected. One of the big reasons for the civil war that racked Japan shortly after the Mongol invasions was driven off was that there simply wasn’t any newly-conquered land to hand out to all the retainers - the war was a defensive one.

No… the institutionalized expectation that a lowly prole should sacrifice “selflessly” for an abstract and immaterial notion such as the nation state is a pretty modern thing - it’s a product of the Enlightenment.