Three Kinds of War

 Three kinds of war.

 Contractual war. War of reprisal, and total, genocidal war.

 Any kind of war not one of these three kinds is a bad war, profane before Mars.

 The idea of a contractual war is to avoid a genocidal war. The two disputants can do things like decide battlefields, forbid weapons, and disentangle civilians. E.g. if civilian power plants aren't used for military purposes, then the aggressor need not blow them up. In a military sense, they are already blown up. The aggressor will likewise agree to some equivalent concession, to avoid painting bullseyes on their own energy supplies.
 The contract is self-enforcing.
 The contract allows one to hold a war and settle a contest without undue costs.
With a pre-defined victory and pre-defined stakes, any resentment the war generates can be safely deemed illegitimate, allowing the dispute to be fully settled and cooperation and trade to resume immediately following the war.

 War of reprisal is used for someone who ought to mind their own business but has mometarily forgotten this. When you don't want to pay for a genocidal war, you can seize some border territories and convert them to a demilitarized zone. Force them to leave you alone.

 If the enemy refuses to limit themselves to contracts, and a limited reprisal does not teach them to keep their hand out of the fire, then it's time for genocide. If they insist on imposing costs beyond their benefits, even given the fact that you've absolutely repudiated the Geneva convention and are openly willing to attempt to wipe them from the planet, then unmistakably the most benefit can be had by causing them to cease to exist.
If a war of genocide is initiated, no surrender ought to be allowed. Destroy them quickly and efficiently, to minimize costs.
Children are just future men, kill them as men. Women are merely potential men, kill them as men. Mercy is unsuitable, unfitting, and unwise. Mercy is vicious, and the victim will be the winner.

 Frequently you'll find enemies unable to follow contracts. (They will often make a virtue of necessity and claim it's due to justice or whatever.) They should be put down like animals, because they are morally equivalent to rabid animals.
 Don't take war brides. It's beastiality.