My point was as a direct counter to your assertion that warfare increases with degree of difference between peoples.

That would predict that the Germans would prefer to ally with other Europeans against people like the Japanese, who they are maximally different from. Historically this did not happen. Yes, there are clear conflicts between each of the groups named, but they are more similar to each other than they are to most other populations of humans.

Let's say an alien combat spacecraft crash-lands in the middle of ISIS-controlled territory. Would you expect ISIS to immediately declare a cease-fire and surrender the craft and its weaponry to the UN so that humanity can work together to face this new threat, or do you think they would immediately go about using it as a tool to better murder their fellow humans?