Martin was appointed as Ambassador to South Vietnam on 21 June 1973 (in 1965 his foster son died in combat in the Vietnam War).

Martin was a controversial U.S. ambassador to South Vietnam; he was ill-prepared to act as an ambassador in a country fighting for its survival.
In 1975, he ignored intelligence and field reports that the North Vietnamese invasion was achieving great success with the fall of provincial capitals.
 He continued to believe that the Army of the Republic of Vietnam (ARVN) would hold Saigon and the Mekong Delta area after observing ARVN's tenacious 12 days of fighting in the Battle of Xuan Loc under the command of General Le Minh Dao.
His delays in initiating an evacuation cost thousands of lives as the options of travel out of the country dwindled down to the final option – Marine helicopters to Navy ships.

In fact, in the NSA history The Secret Sentry, the author says: "In Saigon, Ambassador Graham Martin refused to believe the SIGINT (signals intelligence) reporting that detailed the massive North Vietnamese military buildup taking place all around (Saigon) ... and repeatedly refused to allow NSA's station chief, Tom Glenn, to evacuate his forty-three man staff and their twenty-two dependents from Saigon."
Because of Martin's refusal to believe the SIGINT, and his refusal to allow the evacuation of the intelligence staff from the embassy, "The North Vietnamese captured the entire 2,700 man (South Vietnamese SIGINT) organization intact as well as their equipment."

Martin was evacuated by helicopter from the US Embassy, Saigon on the morning of 30 April 1975 as Communist forces overran the city.
Though he did not know it, the helicopter’s crew had orders to arrest him and bring him on board by force if he had refused to go.
The helicopter used was a USMC CH-46 Sea Knight call sign Lady Ace 09 of HMM-165 serial number 154803.
Ambassador Martin's wife, Dorothy, had already been evacuated by previous flights, and left behind her personal suitcase so a South Vietnamese woman might be able to squeeze on board with her.
Martin was appointed as Ambassador to South Vietnam on 21 June 1973 (in 1965 his foster son died in combat in the Vietnam War).
Martin was a controversial U.S. ambassador to South Vietnam; he was ill-prepared to act as an ambassador in a country fighting for its survival. In 1975, he ignored intelligence and field reports that the North Vietnamese invasion was achieving great success with the fall of provincial capitals.  He continued to believe that the Army of the Republic of Vietnam (ARVN) would hold Saigon and the Mekong Delta area after observing ARVN's tenacious 12 days of fighting in the Battle of Xuan Loc under the command of General Le Minh Dao. His delays in initiating an evacuation cost thousands of lives as the options of travel out of the country dwindled down to the final option – Marine helicopters to Navy ships.
In fact, in the NSA history The Secret Sentry, the author says: "In Saigon, Ambassador Graham Martin refused to believe the SIGINT (signals intelligence) reporting that detailed the massive North Vietnamese military buildup taking place all around (Saigon) ... and repeatedly refused to allow NSA's station chief, Tom Glenn, to evacuate his forty-three man staff and their twenty-two dependents from Saigon." Because of Martin's refusal to believe the SIGINT, and his refusal to allow the evacuation of the intelligence staff from the embassy, "The North Vietnamese captured the entire 2,700 man (South Vietnamese SIGINT) organization intact as well as their equipment."
Martin was evacuated by helicopter from the US Embassy, Saigon on the morning of 30 April 1975 as Communist forces overran the city. Though he did not know it, the helicopter’s crew had orders to arrest him and bring him on board by force if he had refused to go. The helicopter used was a USMC CH-46 Sea Knight call sign Lady Ace 09 of HMM-165 serial number 154803. Ambassador Martin's wife, Dorothy, had already been evacuated by previous flights, and left behind her personal suitcase so a South Vietnamese woman might be able to squeeze on board with her.
Graham Martin