Have you seen The Great War on PBS? One of the episodes touches on this whole aspect of World War I.
They mention that prior to the war, there was no cut and dried “white” population. Most of the U.S. white population were divided up into Irish-Americans, French-Americans, Italian-Americans, etc..
But they were all thrown together in military units and sent off to Europe to die together. When those that survived came home, they had dropped the Irish, French, Italian, etc… self-descriptors. They were all just plain old “Americans”.
I think by the time WWII came around, “race” had evolved to the white/black/Asian concept.
It’s an ugly way to get people to recognize that those “other people” aren’t so different though. As you said, we need to find a way to get there without the need to kill people.