The correct answer is: to protect the rights of its citizens.
Indeed, Thomas Jefferson was a pure product of the Age of Enlightenment which supported the notion of social contract put forward by Jean-Jacques Rousseau and Thomas Hobbes. The social contract clearly stipulates that governments received their authority from citizens (the governed). The latter surrender a few of their rights (the right to use violence to protect oneself in case of aggression is one of them) to them in exchange for the protection of all their remaining rights.
