It's often termed "tribalism," that to which Max is referring, and it remains a thing even to this day, and is also often ideologically tied to defining "us" as "good" and therefore anything "we" do must be "good," and anything "they" do as "evil." Even if they're the same thing. Because "we" are "good" and thus anything "we" do is "good," and those "evil them" are absolute monsters for doing it because, well, they're "evil."

It tends to lead to the possibility of the "father to his men" horribly evil villain-king/warlord. It's one of those things that sometimes seems to complicate alignment debates. In practice, though, it's about what you're willing to do to "them" not in defense of "us," but to "improve" the lot of "us" at "their" expense. That's how you generally determine whether it's really good, neutral, or evil. Genuinely good groups may be willing to do a lot in the name of war, but they do so defensively, or to put down a threat; their goal is rarely if ever to actually gain something (other than peace) that "they" aren't willing to provide.