The ethics of war

To address this based upon the ideals upon which America was founded these questions are relatively easy to answer.

The only "Rights" that exist are the "Inalienable Rights of the Person" and based upon those Inalienable Rights the People have the authority to delegate "powers" to the government. The Individual has an Inalienable Right of Self-Defense Against Acts of Aggression. Based upon this Right the People can delegate the "power" to the government to defend the People from "acts of aggression" by foreign nations. 

The individual does not have a Right to Commit an Act of Aggression against another person and cannot therefore delegate the "power" to commit an act of aggression to their government. 

The government's authority is limited to the territories under it's jurisdiction. In short if another nation violates the territorial integrity of a nation then, based upon the right of self-defense against acts of aggression inherent in the person where the "power" of self-defense is delegated by the people to the government, the government has the authority to defend the People of the Nation from this act of aggression. 

This is actually codified into International Law by Article 51 of the UN Charter:

The UN Charter establishes that if an armed attack against a nation occurs that the nation, or nations, affected have a Right of Self-Defense but that inherent right has it's limitations. First and foremost it does not authorize offensive military operations, only defensive military operations. Next is that this authority only exists until such a time as the UN Security Council has the time to take the necessary measures to maintain international peace and security. 

In simple words, by way of example, if a foreign nation bombs a city in another nation the nation attacked has a right to defend itself from further attacks by shooting down invading foreign bomber aircraft. If it is attacked by artillery then it can, based upon self-defense, initiate counter-battery artillery fire or use it's aircraft to "take out" those artillery batteries. It does not have the authority to invade the country that attacked it as that is an offensive, not a defensive, military response. 

Obviously "preemptive" strikes are expressly prohibited because they are an offensive military action and not a defensive military action. 

As for the morality of combat actions this is also addressed by the Inalienable Right of Self-Defense Against Acts of Aggression the Person. A person can only use the force necessary to stop the act of aggression. The excessive use of force exceeds the Right of the Person. For example a small border conflict cannot justify the use of nuclear weapons in response. The same rule applies to the treatment of enemy combatants that might choose to surrender. They cannot be murdered or tortured as they no longer represent an act of aggression.

Of course in a "post-conflict" situation the "rules" are generally made by the winners as we saw at the end of WW II. Winston Churchhill authorized the bombing of German civilian populations during WW II rationalizing it based upon a single case where a German bomber that had been damaged jettisoned it's bombs over London. London was not a target and targeting of civilian population centers was already a "war crime" prior to WW II under international law. Had Germany won WW II there there wouldn't have been the Nuremberg Trials but instead Winston Churchhill would have been prosecuted for War Crimes and Crimes Against Humanity at the end of WW II for authorizing the bombing of Germany's cities.    

The "winners" of wars always excuse their "war crimes" while prosecuting the "war crimes" of the losers. That has always been the case historically.