Final answer:
The Emperor's main message is to use military force with caution, strategically, and without personal emotion, understanding that the costs of war are irreparable and permanent.
Step-by-step explanation:
The Emperor's main message about the use of military force is one of caution and pragmatism. It emphasizes the importance of making decisions based on advantage and strategic necessity rather than emotions such as anger or desire for personal glory. This can be seen in the directives given to rulers and generals: troops should not be deployed out of personal spite; battles should not be engaged in due to pique; and that anger and vexation can be transient, whereas the consequences of war—such as the ruin of a kingdom or the loss of life—are permanent and irreversible. The enlightened ruler and cautious general are the ones who maintain peace and keep their army intact. These ideas are reflective of the broader attitudes of their times, including the Confucian pacifism of the Song dynasty and the strategic use of force in various historical contexts. The Emperor's advice is to be strategic in warfare, using it only when it is to the country's advantage and always with the utmost caution.