The Battle of FSB Mary Ann occurred when Viet Cong (VC) sappers attacked a U.S. firebase located in Quảng Tín Province, South Vietnam early on the morning of 28 March 1971.
Fire Support Base (FSB) Mary Ann was established to interdict movement of North Vietnamese People's Army of Vietnam (PAVN) and VC troops and materiel down the K-7 Corridor and Dak Rose Trail (branches of the Ho Chi Minh trail running from Laos to the coast of South Vietnam). Originally intended to be a temporary base, it evolved into a more permanent location garrisoned by at least one U.S. Army company. The base was manned by 231 American soldiers at the time of the attack.
The firebase was scheduled to be handed over to the Army of the Republic of Vietnam (ARVN) when the U.S. 1st Battalion, 46th Infantry Regiment moved north to Da Nang. Twenty-one soldiers from the ARVN Battery B, 22nd Field Artillery, along with two 105mm howitzers, were on Mary Ann to support ARVN operations to the south.
For months leading up to the attack the level of enemy activity in the area had been low and contacts were infrequent. The lack of significant recent engagements, along with preparations to turn the FSB over to ARVN units, had given U.S. soldiers in the area a false sense of security. The sapper attack was successful, and was described as a "rampage of VC who threw satchels at the command bunker, knifed Americans in their sleep and destroyed all communications equipment". Army Chief of Staff William Westmoreland investigated the attack, which resulted in repercussions up the 23rd Infantry Division's chain of command, with the probe citing dereliction of duty, lax behavior and failure of officer leadership as the reasons for the attack. Charges were brought against six officers, including the 23rd Division commander and assistant division commander.
Background and base construction
FSB Mary Ann was established on 19 February 1970 by elements of the 1st battalion, 46th Infantry, part of the 196th Light Infantry Brigade (196th LIB). At the time it was not intended to be a permanent base, and was closed by the same battalion about two months after it was opened. Operational needs brought 1/46th Infantry back to Mary Ann on 27 June, and the base was reestablished. According to one battalion commander it was a poor location for a firebase from a defensive standpoint. "[The FSB was built] in a saddle with hills around it on three sides." It was also at the very edge of helicopter range from the Americal Division headquarters at Chu Lai. Any other location in the region would have put the FSB outside the range of the Division's heavy artillery batteries and prevented artillery at Mary Ann from shelling key locations in the region. From an offensive standpoint the FSB was in a good location since it straddled the Dak Rose Trail network.
FSB Mary Ann's construction was no different from many other U.S. firebases in South Vietnam. Running northwest to southeast, the firebase stretched 500 meters along the crest of a ridge connecting two hillsides. Only 75 meters wide at its narrowest point, Mary Ann widened to 125 meters at the northwest and southeast ends. A trench, "knee-deep in certain stretches and waist-deep in others" defined the base's perimeter and connected the base's twenty-two bunkers. Like any typical late-war FSB, most of Mary Ann's bunkers were made from converted metal shipping containers known as conexes. In addition to the conex bunkers, Mary Ann had over thirty assorted structures (hootches, sandbagged bunkers, and other makeshift structures). The southeast end of FSB Mary Ann contained the battalion tactical operations center (B-TOC) and company command post (CP), both located next to a small helipad. The base's mess halls, a communication center, the battalion medical aid station, ammunition bunkers, storage for general supplies, and two artillery firing positions were also located at this end of the FSB. The northwest end of the base contained a second artillery position with two 155mm howitzers, the fire direction center and the artillery command post. The saddle between the two ends of the FSB served as the resupply helipad. Dirt roads divided both ends of the base, and also ran outside the wire at two points: southwest past a firing range to the spring that served as the base's water point and northwest to the trash dump.