Why was leviathan written?

Leviathan was written as a response to both the decline of the church and the decline of the royals. Hobbes was a royalist and anti-clerical. He was struck by how you can't argue with Euclid's geometry. Hobbes believed that individuals were the fundamental unit of political analysis, but nobody had ever written from that point of view before. Up until Hobbes, most people went by Aristotle's 'political animals' theory.