Please enlighten us then. If you don't have data or personal experience to share, do you have anything of value to contribute?
Immigrants to nations have fought for their adopted homes with ferocity in the past. It make sense, really - this is somebody who liked (loved, even) the country so much that they worked hard to get there, as opposed to someone who happened to be born there and so is there by default.
Also, one complication to consider in your example of the American in Germany is that US citizens who join foreign militaries risk losing their US citizenship. Of course, for some people that's not a problem (there are a lot of accidental US citizens), but for others and their families it could pose an issue.
https://travel.state.gov/content/travel/en/legal-considerati...
The first counterexample that came to mind: to be exterminated without even putting up a fight.
I'd judge surrendering your freedom and your property without putting up a fight as "more disgusting than war." Allowing an innocent ally to be conquered probably also qualifies. There's really quite a lot of stuff that is "more disgusting than war." But people can feel free to choose what they're disgusted by...