This suggests surrender rules as an alternative:
If, at the end of combat, one side’s forces outnumber the enemy say 4:1 or greater they are considered to have taken control of the region and the surviving enemy units are taken prisoner.
When a nation is eliminated/collapsed it must release all POWs held which are sent to the original owner as per purchased units, assuming that nation is still active.
Should probably include land units only.
Yes, it eliminates the idea of a single courageous unit digging in to hold on against overwhelming odds, but this was a war of numbers and greater numbers eventually told even against entrenched positions. Is it realistic for a single Italian unit to hold up the entire Austro-Hungarian army in Venice? This is how Hitler and Stalin expected soldiers to behave in 1941-5, but is not authentic for 14-18.