The present paper suggests that the recurring appeal to kinship diplomacy undermines a fixed idea of ‘nation’ in Archaic Greece, especially in the first two decades of the fifth century BC. It aims to present a series of test cases in Herodotus that explain why contemporary patterns and theories on ancient ethnicity can hardly explain the totality of the historical spectrum. Blood ties could sometimes fortify ethnic relationships, as in the case of Aristagoras’ mission to Sparta (Hdt. 5.49.3), since the common Greekness could elicit the Spartan to help to the Ionians. In other times, the same blood ties were applied to divine genealogies, and they could also be used to show the feeble devotion of cities like Argos to the Greek cause (7.150.2: Xerxes expects the Argives to join the Persian cause, since they descend from Perses). Habits and traditions, often taken as indicia of national feeling, could be thought of as clues of ancient migrations (so the Trojans became Maxyes in Lybia: 4.191). Even language might not help in justifying ethnic relationships: for instance, the Greeks living in the Scythian Gelonus spoke a mixed language (4.108). These few case studies may shed a different light on the classical definition of Greekness (to hellenikon) in terms of blood, language, cults, and habits, all given by Herodotus (8.144). Far from being a valid label for all the Greeks of the fifth century, this statement owes much to a specific variety of the language of kinship diplomacy. The final section argues for the opportunity to avoid the later and misleading idea of nation when studying Herodotus and the age of the Persian Wars, which are instead characterized by various and contrasting strategies. Greek groups and ethne can be better described as networks of lightly defined communities.