While in Vietnam his unit provided artillery support to the Marine infantry units who were engaged in the June 1967 battle of Khe Sanh. During the battle, Brennan was wounded multiple times for which he received the Purple Heart. Disregarding his wounds, Brennan remained with his unit, encouraging his men and directing return fire, which completely destroyed the opposing forces. He then ensured his fellow wounded soldiers received medical attention before allowing himself to be treated. For his actions Brennan was awarded the Bronze Star. After completing his tour in Vietnam, Brennan was selected for Amphibious Warfare School in Quantico, Virginia and was subsequently promoted to Major in November of 1967.
Under President Richard M. Nixon, Brennan was nominated and selected to be the first Marine to hold the position of Marine Corps Aide to the President. As President Nixon’s military aide, one of his duties was to "carry the football" or nuclear codes to initiate the launch of nuclear missiles in the event of a war. Later when President Nixon left the office of the President in 1974, Brennan accompanied the President to California to continue as his aide. The following year, Brennan left the Marine Corps to become the former President's Chief of Staff until 1980 when he started his own consulting business with the former Attorney General, John Mitchell, in Washington, D.C. (continued)