It's not nationalism per se that is the most destructive force of the 20th century it was nationalism combined with SOCIALISM.
Nazis were national SOCIALISTS.
The Soviet and Chinese et al Communists were socialists who practiced the most brutal sort of ethnic, nationalist bigotry imaginable. SOCIALISM-at-the-point-of-a-gun.
That's the only way to fly for socialists in the 20th century - combat nationalism (as in the pride of the British, French, USA in WWII). Fascists with the Big Socialist Plan always hate people who speak of national pride in values and special culture traits.
All must be consumed by the Leviathan, you see. Especially you. And yours.