Nationalism, in the political philosophy sense, holds that people owe special, genuinely stronger obligations to fellow members of their own nation than they owe to people elsewhere, obligations of solidarity, mutual support and shared political priority. Cosmopolitanism holds instead that basic moral obligations extend equally to every person regardless of nationality, treating national membership as morally arbitrary in the way that being born in a particular place shouldn't determine how much moral consideration someone is owed. The two views disagree sharply about whether national borders carry any genuine moral weight of their own.
Nationalism grounds special obligation in genuine shared membership and history
Nationalist political philosophy typically grounds its claim of special obligation in something real about shared national membership, a common history, shared institutions, mutual cooperation and genuine interdependence among fellow citizens, arguing that these real, substantive bonds justify treating compatriots differently from strangers in a way pure abstract moral reasoning alone wouldn't require. On this view, national borders aren't simply arbitrary lines; they mark out communities bound together by real ties that generate real, additional obligations beyond the baseline obligations owed to people generally.
Cosmopolitanism argues those same ties don't actually justify unequal moral standing
Cosmopolitan philosophy challenges that reasoning directly, arguing that being born inside a particular nation's borders is itself a matter of pure chance, and that basing genuinely different levels of moral consideration on that chance circumstance is difficult to justify from an impartial moral standpoint, the same standpoint that would reject treating people differently based on other, similarly arbitrary facts about their birth. On the cosmopolitan view, shared history and cooperation might justify some practical, institutional differences in how obligations get organised and delivered, but they don't justify treating a stranger's basic moral claims as genuinely weaker than a compatriot's simply because of which side of a border they happen to have been born on.
What we're still unsure about
That nationalism and cosmopolitanism offer genuinely opposed, carefully argued answers to whether national membership carries independent moral weight is well established, extensively debated territory in political philosophy. What remains a genuinely unresolved, actively contested question is exactly how much weight, if any, shared national membership should carry once its moral relevance is granted at all, since even philosophers broadly sympathetic to cosmopolitanism often accept that some special obligations to compatriots are reasonable, and the field continues to actively debate where a defensible middle position, if one exists, should actually sit between pure nationalism and pure cosmopolitanism.
This sits inside Nationalism & Cosmopolitanism, one of seven topics in Political Philosophy, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.