The rule of warfare includes: not to attack the troop in the highlands, not to fight the enemy with hills behind him.
Not to pursue soldiers who pose to retreat, not to fight the enemy whose spirit is on high.
Not to take allurements offered by your opponent, not to interfere with residual forces in retreat.
When besieging your enemy, leave a corner open (allow him to evacuate). Do not push defeated yet desperate soldiers too hard.
All these are the rule of warfare.