Sergeant Frederick Brunner was officially assigned to the Marine Detachment at the US Embassy in London. However, in reality he was assigned to the Office of Strategic Services (OSS), the forerunner of today's Central Intelligence Agency.  In August of 1944, he was parachuted behind enemy lines into France where he trained French resistance units on the use of advanced weapons.  For his service in France, he earned the Silver Star (citation below.)  In March of 1945, he was to be parachuted into Germany as part of an OSS operation to gather information and report on German troop movements and political developments.  However, he died when the plane that was taking him to Germany crashed in bad weather.  (This was the same plane crash that killed Kurt Gruber who is also buried at the Ardennes American Cemetery.)

Brunner's Silver Star citation reads:

The President of the United States of America takes pride in presenting the Silver Star (Posthumously) to Platoon Sergeant Frederick J. Brunner (MCSN: 371861), United States Marine Corps Reserve, for conspicuous gallantry and intrepidity as an Arms Instructor serving with Marine Detachment, American Embassy, London, England, in action against enemy German forces in France, from 1 August to 3 October 1944. Parachuted behind the German lines into France to instruct the Beaufort Maquis and other resistance groups in the use of modern weapons, Sergeant Brunner skillfully trained these guerilla forces to a high state of efficiency and, participating in their audacious forays against the Wehrmacht, disrupted communications and troop movements and assisted in the liberation of numerous towns in the department before arrival of Allied troops, On 16 August, when his patrol was dispersed by a numerically superior enemy force, he calmly returned accurate fire as he effected a withdrawal, swam across a swiftly flowing, bullet-swept river without loss of arms or equipment and, risking execution as a spy by donning civilian clothing, made his way fifteen miles through German-controlled territory to regain contact with his headquarters. By his courageous actions, resourcefulness and devotion to duty, Sergeant Brunner contributed materially to the success of the Allied Campaign in Europe, thereby reflecting great credit upon himself and the United States Naval Service.