Shinigami ni Sodaterareta Shoujo wa Shikkoku no Tsurugi wo Mune ni Idaku - Vol. 3 Ch. 14 - An Official Appointment.

[14Darius28]

You would think negotiating a prisoner exchange would be the only thing to do with prisoners of war. Their really stretching it for us to believe that no one in there history has ever even thought of a prisoner exchange

It's not that he didn't think of it or no one thought of it. It's that they don't normally do it for common soldiers. In chess that would be like kinging a pawn into an extra pawn. It's not normally done so it's not something command was prepared to hear. Ashton on the otherhand decided on it because the kingdom has been beat back so bad that they have to use conscripts to keep a line.