This dissertation analyzes four defenses of a Greek upland ethnos ("people," "nation," "tribe") against a large-scale invasion from the lowlands ca.490-362 BC. Its central argument is that the upland peoples of Phocis, Aetolia, Acarnania, and Arcadia maintained defensive strategies that enabled wide-scale, sophisticated actions in response to external aggression; however, their collective success did not depend on the existence of a central, federal government. To make this argument, individual chapters draw on the insights of archaeological, topographical, and ethnographic research to reevaluate the one-sided ancient narratives that document the encounters under consideration. The defensive capabilities brought to light in the present study challenge two prevailing paradigms in ancient Greek scholarship beyond the polis ("city-state"). Beyond-the-polis scholarship has convincingly overturned the conventional view of ethnē as atavistic tribal states, emphasizing instead the diversity of social and political organization that developed outside of the Greek polis. But at the same time, this research has emphasized the act of federation as a key turning point in the sociopolitical development of ethnē, and downplayed the role of collective violence in the shaping of upland polities. In contrast, this dissertation shows that upland Greeks constituted wellorganized, efficient, and effective polities that were thoroughly adapted to their respective geopolitical contexts, but without formal institutions. Furthermore, the defensive strategies of these polities reveal that upland Greeks formed part of a connected sphere on the classical mainland, but in a surprising sense. While the more powerful states of the lowlands were arrogant, ignorant, and dismissive toward the collective capabilities of upland peoples, the reverse was not the case. The Phocians, Aetolians, Acarnanians, and Arcadians were well aware of the dismissive attitudes of their aggressive neighbors, and exploited them to great effect.