The History of Thucydides is concerned with the causes and the events of the Peloponnesian War: a time of great strife and conflict and, furthermore, a time when, perhaps as never before or since, the decisions and actions taken by large groups of people gathered together was of paramount importance. The classical poleis were at their height, and every one of them was ruled by a larger or smaller group of people. Athens was governed by a democracy more direct than our own; and the other players in the war, be they democracies or more oligarchic, were alike subject to policy formed by groups of men. The crises of such a large war threw the strengths and weaknesses of this system of collective government into a starker light; and the war itself also meant that large bodies of men, the armies, were gathered together usually in times of overwhelming pressure. Thus, to a great extent the fates of the poleis were determined by people operating collectively; and it is therefore apparent that the nature and effects of collective psychology were of great importance to the War and to the historian who recorded it. Even the conventions of language in the History serve to remind us of the collectiveness of decision-making, for the players in the war are not referred to by the names of their poleis, but, rather, by a collective noun for the inhabitants: thus not "Athens mobilised her armies", but "the Athenians ...". The War was a clash of crowds and armies, rather than carefully thought-out campaigns produced by single sovereign minds.
This dissertation seeks to describe and analyse Thucydides' notions about collective psychology; what he thought were the characteristics of the multitude, both civil and military, when they were all gathered together and the effects this could entail; how he envisioned the role of the leaders of the states, and the interaction between them and the masses.