Before there were nations there were empires. Nations are an improvement on that. The word ‘nation’ is a term for a political polity. Actually it could be of any size or composition, or called something else, like ‘corporation, fubar, commune or word-factory. I like Karl Popper, yet the paradigm of national socialism (NAZI) is not the paradigm of every nation. That is it is not the sole form of content a nation could take. A nation may be regarded as a polity where people share the same values freely (if it is a Democracy). If there were just one global nation then everyone probably would be forced to share one value system. There are innumerable other problems with one global nation I won’t digress into here.
A nation or a home, an organization or government, a business and any coherent cell need have practical working boundaries in order to exist. One may of course choose anarchist, chaos or anomic values or those of any moral system inconsistent with stable national cellular health. There is wild freedom after all. Some arguments are fit for a particular time, while others-Platonic political paradigms or those of Aristotle, might be regarded as more Universal. Popper’s paradigm applied today would be closer to an anti-Semitic, Nazi hatred of Israel, although that position would contradict Popper’s sentiments.
Popper was something of a reactionary writer describing the aftermath of the Second World War. He incorrectly assumed that nationalism and populism were the real causes of the war – in addition to race and nationality. In my opinion the causes were original sin and aristocracies of the former Weimar Republic era that wanted a way to get back their lost royal prerogatives. Hitler and the national Socialists were the baby-mama surrogates for them.
Popper was viewing a particular phase of human history; its demographics and evolution and incorrectly saw nations and nationalism as the cause of conflicts. It was far more complex than that. The content or political composition of many nations was inefficient, repressive or unable to play well with other nations in the early 20th century. Today mass communications and travel have changed that. In the present nations are the best political cells where the experiments of democracy can occur. They are something like what the several states were in the U.S.A. at its foundation, where differences were allowed and central government was not completely dominant and repressive of local self-determination.
Nations can defend the citizens from hostile takeover by one adverse global central power; repressive minority power is the usual suspect in the denial of independence, free enterprise and self determination. The threat today is the axis of corporatism and communism that would make proles of everyone.