Which philosopher wrote Leviathan in 1651?

Thomas Hobbes, writing in exile in Paris during the English Civil War - which is the whole point of the book. He had watched a state collapse into competing armies and concluded that almost any sovereign is preferable to none, because without one, life is solitary, poor, nasty, brutish and short.

The argument is a contract rather than a divine right, which is what made it dangerous to everyone. Hobbes derives absolute authority from self-interest: rational individuals surrender their freedom to a sovereign because the alternative is a war of all against all. That justified the king to nobody's satisfaction - royalists objected because it grounded authority in consent rather than in God, and parliamentarians objected to the absolutism. He was accused of atheism and the book was later burned at Oxford.

It is also the origin of a phrase everyone half-knows. The state of nature is a thought experiment, not a historical claim - Hobbes was describing what happens when authority fails, and he had seen it. Rousseau later inverted the picture with a state of nature that was peaceable and a society that corrupted it, and the argument between those two accounts of what people are like underneath is still the load-bearing disagreement in political philosophy.