There’s a lot of circular reasoning here, because winning wars are very, very popular. We didn’t enter WWII because of high-minded principles like the 4 freedoms and the UN — the public supported entry into the war only after we were attacked. The Gulf War was popular because we won decisively, and quickly. The Iraq war was popular until after we captured Saddam, and started losing. Then not so popular. Vietnam? I still remember as a small boy seeing the “Victory at Sea” headline in a New York paper in the days after Tonkin Gulf skirmish. We had few soldiers there then, but the war was popular. The more soldiers we sent, the more elusive victory became, and support for the war declined steadily.
So I don’t think sustained public opinion is the measure of a just war.
Sometimes I think we mythologize the Civil War because its soldiers wrote some beautiful, inspiring letters that leave us so impressed with their nobility. I wonder if you could find similar letters by the soldiers just before and after the Civil War, when the death struggle was with Native Americans and not Southern slaveholders. If those frontier war letters were equally beautiful — would that war then be considered blessed also?