Out of the loop

You ever played video games like Stellaris, where galactic civilizations gain bonuses for declaring other civilizations their rival?

This is that, but in real life. By being antagonistic, each nation can gain a citizen production bonus by taking advantage of their nationalistic pride. These citizens are then more likely to seek out and purchase products made in their own country, thus recycling the money back into their own economy, giving an economic bonus as well.