The pressure of war often drives change. This was no less true of the Peloponnesian War in its effect on constitutional thinking at the end of the fifth century. While Thucydides in his analysis of the Peloponnesian War suggests that it was differences between constitutional types that lay behind the conflict (democracy versus oligarchy), it was in fact the war that clarified these differences. Thus it was that ideas around democracy became more clearly defined. However, it was thinking about oligarchy which experienced the most radical changes. Earlier in the fifth century, oligarchy had been recognised as a constitutional form but had been fairly loosely defined. By the end of the war, however, some Athenians in particular, who wanted to effect regime change, played with ideas of oligarchy in a fairly imprecise way based on number, wealth or class. Initially, this lack of clarity worked in the favour of the reformers, but eventually it led to the downfall of both the oligarchies of 411 and 404/3. Nevertheless it was the war itself which ultimately forced the conceptual opposition between oligarchy and democracy, which Thucydides was then able to write retrospectively into his analysis of the Peloponnesian War.