In his Discourses Concerning Government, Sidney undertakes to vindicate human liberty against the claims of Robert Filmer, whose Patriarcha maintains that nature and God dictate human dependency. Human beings, in Filmer's view, are born subject to a rigidly hierarchial chain of command that passes upward from the father, to the king, and finally to God. On the way to his repudiation of the Filmerian divine right of kings, Sidney produced the diffuse Discourses, which, among many other topics, considers the corrupting influence of monarchy, the unreasonable character of inheritance as a claim to rule, and the vigor of republics.

Liberty, it becomes increasingly clear as his Discourses progresses, is best guaranteed through a republic. He acted on that belief throughout his life, although it has been said that his commitment to republicanism was “intermittent and inconsistent.” He fought on the side of Parliament and was wounded in battle at Marston Moor in 1644. He entered Parliament in 1646, where he somewhat surprisingly – given the glee he will later manifest in his Discourses when contemplating the punishment of wayward monarchs – opposed the execution of Charles I. He came to work closely with a group of republicans in Parliament, among whom was Henry Neville, whose family had shared friendships and alliances with his own that went back for generations. He actively supported England's efforts in the Anglo-Dutch war, which broke out in the middle of 1652, and he joined the Council of State later that same year.