I have read ALL of Heinlein's work's. He was more philosopher than writer. His observations of the human condition hit the bullseye of the modern world, be it political, social, religious - pick a topic. This particular quote obviously strikes a chord and makes people think. Peace AND Freedom are both Transient conditions. Freedom for each and all can only be maintained through a constant readiness to defend it, by violent means if necessary. Peace can be maintained by simply giving up freedom, if that's the least objectionable path; or a brief term of peace MAY be attained as the result of choosing to have freedom. But remember, History is always written by the victors.