Mrs. Lawhawk and the Rules of War

This past weekend, Mrs. Lawhawk and I watched a bunch of movies. Dr. Zhivago, The Great Escape and Bridge on the River Kwai. David Lean classics. It was the first time the Mrs. had seen Zhivago and she knows that Kwai and the Great Escape are some of my favorite movies. There's a bit of a theme among those movies, besides being great movies that feature actors like Sir Alec Guiness, Omar Sharif, or Steve McQueen and John Sturges and David Lean as directors.

In the course of watching these movies, she had a question that had me scratching my head in wonderment and bemusement.

She asked:

Why do countries keep POWs instead of simply killing anyone on the battlefield? Wouldn't it simply be easier to kill them than to devote all kinds of resources for prisoner camps, which could otherwise go towards the war effort?

Damn fine question.

I couldn't quite give her an answer to my own satisfaction, let alone hers. The laws of war, the Geneva Convention, and prisoner of war status are all recent inventions, but they don't quite get to the heart of the matter - why countries would bother to take prisoners in the first place.

What do they get out of taking prisoners. Not every prisoner is valuable for intel purposes, and there's diminishing returns for propaganda purposes, so why do countries still engage in this activity during wartime. Mrs. Lawhawk awaits a concise response.