Lyndon Johnson's 1965 speech given at Johns Hopkins University was a justification of his choice to escalate the United States involvement in the war in Vietnam. The lines I think you are referring to are: "This is the principle for which our ancestors fought in the valleys of Pennsylvania. It is the principle for which our sons-fight tonight in the jungles of Vietnam." The first major answer he gives to his question of why we were fighting in a foreign war against a country that was no immediate threat to the United States was "We fight because we must fight if we are to live in a world where every country can shape its own destiny. And only in such a world will our own freedom be finally secure." This suggest a principle that freedom is the highest good and that it can only be preserved by a stalwart defence. The specific freedom is that for "a nation to choose its own destiny". Johnson additionally assumed something known as the "domino theory", that if one country fell to communism other would follow.