Or, alternatively, this is a good time to test loyalties and weed out the bad apples.
if just half the 15% turned on their comrades and took one out, they would lose 15% of their forces outright, half of that to casualties and half to desertion+joining enemy theoretically speaking. an army that loses 20% is no longer combat effective, in theory (the marines proved that wrong in iwo jima but this is the french, no offense intended)