Japan and Germany learned it the hard way because we bombed Dresden and Tokyo to the ground, indiscriminately killing hundreds of thousands of civilians on purpose, not even targeting manufacturing centers let alone military targets. We've murdered entire villages of Vietnamese. But it's ok because it was us, and that wasn't reflective of who we are as a people. But if Russians commit atrocities in war then it is.
Who said it was okay? You assume too much. The option to not bombing those cities was the potential thousands of dead American service members by invasion.
Do you need reminded who started these conflicts? Japanese and German war crimes are amongst the most expansive in recorded history. Nanking in particular was exorbitantly brutal even by medieval standards. American war crimes in Vietnam were adjudicated and tried by court martial. Nobody is saying we’re perfect, but we have standards. Did you not watch the Gallagher fiasco recently? The Hague and Geneva conventions codified the basic principles in the law of armed conflict. Over 190 countries agreed on or ratified these standards - Including the former USSR/Russian Federation.
Modern western militaries hold their service members accountable - Russia doesn’t. War is hell, but your appeasement and counter of Russian aggression is very Chamberlain-esque. Your entire debate centers around “The U.S. has done worse - so it’s okay for the Russians to do.” …How you logically keep that together is some twilight zone level mental gymnastics.