War is ethical if the reason behind the war is morally righteous
If all other peaceful options have been tried and have failed, then a war may be an ethical next step. The war is only ethical if the reason for going to war is virtuous.
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If war is waged as a result of some injustice needing to be righted, then it is an ethical war. The U.S. entered World War II only after Japan bombed Pearl Harbor. After Pearl Harbor, the U.S. had two options: sit idly and not retaliate, allowing Japan to get away with the attack and possibly carry out more, or declare war and keep American citizens safe. Thus, the U.S. entered the war ethically. When they entered the war, they only declared war against Japan. That was the only country who the U.S. needed to go to war with to right the wrongs of Pearl Harbor. It was Hitler and Germany who then declared war against the United States, leaving the U.S. no option but to defend itself. In instances like these, where war is entered because of a blatant and deadly act of aggression by another party, the war is ethical. Further, when a country goes to war out of the necessity to defend it's citizens, that too is a morally righteous and thus ethical war.
While the attack on Pearl Harbor was unprovoked, it is worth noting that rather than talk about it civilly, the U.S. decided to declare war on Japan. This leap in action takes away from the degree to which the U.S. ethically entered the war. There was no evidence to show that Japan intended on attacking again. The more righteous-- and thus more ethical--action to take would have been to diplomatically discuss the attack and come to an agreement that corrected the wrongs without going to war. Most wars are entered because a perceived injustice. Many wars fought in the Middle East were fought because the United States' lack of oil was an injustice. Those wars were far from necessary, let alone ethical.
[P1] If the reason behind the war is righteous, the war is ethical. [P2] If war is entered to right an injustice, it is ethical.
Rejecting the premises
[Rejecting P1] Conflict can be solved diplomatically, even if it starts violently. [Rejecting P2] Injustice is a term that is subject to many definitions.