Battle are fighted into a frame of a year (one turn).
This doesn't mean they last so long. While
"Prisoner of war" is a concept I like and already debated, I agree that the concept of capturing a useful unit is a bit oversimplified.
Of course weapons and supplies can be captured (e.g. horses or working trucks and tanks);
some units can be captured under particular limits (modern ships inside harbour, planes on hangar/airfield).
It's true that ships as trireme or galleons where captured also on the sea (but not modern ships, usually), while some plane has been hit and captured in good condition after emergency landing.
I simply suggest to use the Razor of playability vs. realism, this time.
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Admiral Naismith AKA mcostant