Classic theories of nationalism, whether modernist or ethnosymbolist, emphasise the role of elites and spread of a common imagined community from centre to periphery. Recent work across a range of disciplines challenges this account by stressing the role of horizontal, peer‐to‐peer, dynamics alongside top‐down flows. Complexity theory, which has recently been applied to the social sciences, expands our understanding of horizontal national dynamics. It draws together contemporary critiques, suggesting that researchers focus on the network properties of nations and nationalism. It stresses that order may emerge from chaos; hence, ‘national’ behaviour may appear without an imagined community. Treating nations like complex systems whose form emerges from below should focus research on four central aspects of complexity: emergence, feedback loops, tipping points and distributed knowledge, or ‘the wisdom of crowds’. This illuminates how national identity can be reproduced by popular activities rather than the state; why nationalist ideas may gestate in small circles for long periods, then suddenly spread; why secession is often contagious; and why wide local variation in the content of national identity strengthens rather than weakens the nation's power to mobilise.