War is ethical if the response is proportional to the initial act of aggression
If actions taken in a war are reasonable responses to real or perceived aggression, then the war is ethical. It is being waged with intentions that are not meant to cause unnecessary casualties.
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A war is ethical if it prevents more suffering than the war itself causes. For example, World War II helped prevent the mass execution of the Roma, homosexual, and Jewish communities in Europe. Without the intervention of the United States, Germany and the Axis may have won. This war again the Axis Powers was thus ethical, as the war caused fewer casualties than would have occurred if there was no war. This "proportionality doctrine" is a way for nations to act in both self defense, and in defense of others. The proportionality doctrine gives nations a general set of guidelines to follow if they are to enter into war in an ethical manner. For example, it would be ethical to respond to an air raid on a group of ships with an air raid on enemy ships, but it would not be ethical to respond with a nuclear missile. As long as actions are proportional to one another, the war is being fought ethically.
The proportionality doctrine is fine enough in theory, however in practice it leads to escalation. It may prevent large leaps in escalation in the short term, but repeated acts of aggression from each side will slowly raise the stakes. What if an air raid on munitions factories accidentally level a neighborhood, killing civilians? Would the proportionality doctrine allow for retaliation in the form of more civilian casualties? The proportionality doctrine allows for mistakes and accidents to turn into "justified" war crimes for each side, which are unethical.
[P1] War is ethical the acts of war on both sides are comparable to one another. [P2] War is ethical if the war is fought in self defense or in defense of others.
Rejecting the premises
[Rejecting P2] Proportional acts of war only serve to slowly escalate conflict.