The various evolutionary adaptations that various human races have developed are a successful means of dealing with a particular enviornment and climate.

The Asian skin folds to ward off the wind
The white skin to produce the maximum vitamin D from minimum sunlight
The black skin to stop penetrating equatorial sun and hide from prey
etc.

When an outsider comes to interbreed and alter those adaptations aggresion is one means to prevent the dissolution of those special life saving traits.

In a modern civilation the various traits have little or no advantages whatsoever.

But the reptilian mind of agression and self/tribal preservation lives on.

Add nuclear weapons to the mix and you can see what is coming.