Half-measures create sequels. A rival left half alive rarely becomes humble; they become patient and resentful.
If you choose conflict, finish it decisively. Remove resources, isolate allies, deny legitimacy, and close the routes they would use to return. The goal is not cruelty. The goal is preventing revenge.
Do not strike in a way that creates a martyr. A martyr unites people. Instead, end momentum quietly: make the enemy irrelevant, discredited, or displaced from your territory. Sometimes the cleanest ending is a face-saving exit that moves them far away.
An unfinished enemy waits for your weak moment. A finished enemy cannot. Decide carefully before you strike, because total conflict has a cost. But once you strike, do not leave loose ends.