Liberty is the political freedom of an individual. It is one of the "unalienable rights" listed in the Declaration of Independence. America's core political values, called republicanism, are based on the extension of liberty to all. Liberty has been a powerful force in politics since the early 1800's, as governments around the world saw the success of the American model and have attempted to emulate it.
JFK said, "Let every nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden, meet any hardship, support any friend, oppose any foe, in order to assure the survival and the success of liberty."