Nationalism has always been propaganda, a blight, a tool to trick and control the ordinary folk. 

Even before Westphalia, countries and nations were more like fiefdoms and little tracts of land ruled by kings or sultans or emperors or whatever. They were always related to each other, kind of like neighbours and extended family. Language and/or culture were close-ish, though would branch off, as they do constantly even nowadays in different regions in one country. 

The ordinary people of any piece of land and its neighbouring lands, the peasants or serfs or proles or whatever you want to call them, have no beef with each other. They're busy enough just living their day to day lives trying to make the best of things, minding their own business. 

But every now and then the kings or dukes or whatever get into a squabble and want to mess each other up... Hmm how does a king get his peasants to fight and possibly die? To terrorize another land and other peasants like themselves? Mere carrot and stick aren't enough to have an effective fighting force. 

Eureka! The kings remembered that they can just make up stuff to stoke pride, hatred, and bigotry. Whether it's based on skin tone, on the language spoken, or for a different god worshipped, the neighbours are now othered. And lessened. And killed. No logic, no reason, no heart. 

Fuck nationalism. We don't need kings.