Troops Will Be Powerful When Fighting a Just Cause.
This term suggests that when there is a good cause to use military force, the troops will be high in morale and valiant in fighting. The word shi (师) here is a general term for all military forces and operations. The word zhi (直) means a just cause. The word zhuang (壮) means powerful troops. The Chinese nation has always been wary of waging wars, believing that a war should be fought only for a just cause and that an army fighting for such a cause will have high morale and wins the war.
In fighting a just war, the troops will have high morale and be powerful, while they suffer from poor morale when the cause is unjust. Morale has nothing to do with how long an army is deployed at the front. (Zuo’s Commentary on The Spring and Autumn Annals)