Humanity is tribal. We are social and cultural animals. Culture lets us co-operate not just in family bands, but in imagined communities. Of all such communities nothing is closer to family than “nation”, a word signifying shared ancestry.
The capacity to create imagined communities is humanity’s strength and among its biggest weaknesses. Imagined community defines what people share. But what binds them together divides them from others. Today, as in the past, leaders foment aggrieved nationalism to justify despotism and even war.
For much of human history, war was seen as the natural relationship between societies. Victory brought plunder, power and prestige, at least for elites. Mobilising resources for war was a core role of states. Justifying such mobilisation was a core role of culture.