Some forces never retreat from combat, no matter how bad it gets.
Where your detachment has the No Retreat ability:
After you lose a close combat or firefight with the detachment, do not make a Retreat move with it. Units in your detachment that remain within 15 cm of enemy units are not destroyed.
[TODO: We should add something about what happens if a No Retreat unit starts a turn in base contact with the enemy, as at FAQ: Starting the turn in contact with the enemy]