Nationalism holds that humanity is divided into nations, that each is entitled to govern itself, and that political and national units should therefore coincide. Stated that plainly it is among the most successful political ideas in history, since nearly every state on earth now claims legitimacy in its terms, and among the most contested in what it implies.
Its intellectual sources pull in different directions. Herder supplied the cultural version: peoples are constituted by language, poetry and inherited ways of life, each valuable in itself and none rankable against the others. The French Revolution supplied the civic version: the nation as the body of citizens in whom sovereignty resides, membership defined by allegiance and not by descent. The distinction between civic and ethnic nationalism descends from that difference, though critics note it flatters the states claiming the civic label, since every civic nation has a dominant culture its members are quietly expected to assimilate into.
Scholarship has largely inverted the tradition's self-understanding. Ernest Gellner argued that nationalism produces nations and not the reverse: industrial society requires a mobile, literate, culturally standardized population, and mass schooling manufactures the nation it claims to express. Benedict Anderson located the mechanism in print capitalism, which let strangers conceive of themselves as contemporaries in a single society. Anthony Smith pushed back, arguing that modern nations are built from real pre-modern ethnic materials and not conjured from nothing.
Its record is genuinely double. Nationalism drove decolonization and remains the principal argument available to peoples under foreign rule. It also underwrites expulsion, partition and genocide, because a doctrine requiring political and national boundaries to coincide implies moving one or the other. National conservatives such as Yoram Hazony argue the nation-state is the only proven vessel for self-government and mutual obligation. Their critics reply that the boundary is always drawn by someone, and rarely by those it excludes.