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.
Soon after being reopened, FSB Mary Ann was probed many times (four attempts are recorded between July and August 1970) and one author states that the base could have been easily observed from the high ground surrounding its location. The last major contact in the area was a firefight on 13 August, when Company A, 1/46th Infantry hit and overran "what was probably the NVA command post for the area." After that firefight, organized resistance seemed to cease. That quiet, combined with the usual cycle of replacements at all levels of American units (from individual riflemen to the company and battalion commanders), led to what the outgoing battalion commander called an "awfully complacent" mindset.
Conditions prior to the attack At the time of the attack, FSB Mary Ann was garrisoned by Company C, 1/46th Infantry, a 75-man unit commanded by Captain Richard V. Knight. In addition, 18 men from Company E's Reconnaissance Platoon were at the base preparing for an operation the following day. These troops shared space with 34 support personnel (medics, radiomen, etc.) from 1/46th Infantry's Headquarters and Headquarters Company (HHC). The rest of the garrison (less the ARVN artillerymen) came from elements of A, B, and D Companies, 1/46th Infantry (22 men who were in transit between their units and areas further to the rear), and assorted artillery personnel (including the crew of a quad-.50 caliber machine gun from Battery G, 55th Air Defense Artillery Regiment). According to one source all ground surveillance radars and night vision (starlite) scopes formerly at Mary Ann were "sent to the battalion rear for maintenance." This left the base with a series of ground sensors designed to detect movement located "within fifteen hundred to forty-eight hundred meters" of the perimeter. These sensors had been picking up movement since shortly after the base was reopened, but no contact had ever resulted from the readings.
Company C was the only complete company at Mary Ann due to operations south and east of the FSB near a location called Landing Zone (LZ) Mildred (). Companies A and B, 1/46th Infantry, were on the ground in that area of operations, and some of the artillery previously located at Mary Ann had been moved to LZ Mildred to support operations there (including the 81mm mortars assigned to both companies and the heavy 4.2-inch mortars normally part of Company E). Prior to the attack, the attention of the battalion commander, Lieutenant Colonel Doyle, was focused more on operations near LZ Mildred. In fact, the battalion command post was scheduled to move there on 28 March. This impending move led to a freeze on all new construction at Mary Ann, including fencing to block the roads leading out of the FSB.
Early accounts of the attack state that the defenders "failed to safeguard the perimeter" of the FSB. The only single volume study written about the attack by Keith Nolan contests this position, pointing out that "historians [including the author of the book, who had discussed Mary Ann in an earlier work] got it wrong." Even with this new perspective, there are indications that alertness at Mary Ann was not as good as it could have been. Many accounts by men stationed at Mary Ann mention that perimeter security was uneven at best. One member of the 1/46th Infantry Headquarters Company stated that "I don't think Captain Knight had a clue as to how lax the security was ... because in that area, and only that area, Knight wasn't diligent. He left it up to his lieutenants and sergeants to check the bunker line." In many cases they failed to do so, and the failure was compounded by Company C being understrength and unable to man all the perimeter bunkers. Manning bunkers was not the only problem. Tripflares located in the Concertina wire around the base were not always reset or replaced when they went off, a common occurrence caused in some cases by the rotor wash created by large CH-47 Chinook helicopters bringing in supplies or backhauling material as 1/46th Infantry prepared to hand the base over to the ARVN. This lax attitude, combined with the skill of the attacking sappers, had fatal consequences for many in Company C. Although not "cringing in their bunkers", the defenders of Mary Ann were not prepared for a ground attack. This lack of readiness was not noted by the 196th LIB commander, Colonel William S. Hathaway, during a visit to the base on 27 March, the day before the attack. He later stated that what he had seen at Mary Ann that day "was a big improvement over that I had seen before ... the troops were alert."
Battle In the predawn hours of 28 March sappers from the VC 409th Sapper Battalion approached the wire of FSB Mary Ann and took up positions to launch an attack. The exact number of sappers involved is uncertain, but most sources agree that there were at least 50. As was common practice for such units, the sappers wore khaki shorts and soot camouflage and were armed with either an AK-47 or RPG-7 and satchel charges and grenades to attack bunkers. Sappers relied on stealth, shock and surprise to give them an advantage, and rarely carried heavy weapons or equipment. The 409th was known to the intelligence section of the 196th LIB, but had previously operated against ARVN targets north of Quang Tin Province. It was assumed by the 196th LIB's intelligence personnel that both the 409th and the 402nd Sapper Battalions were east of FSB Mary Ann, preparing to attack ARVN targets in that region. None of them predicted an attack on the U.S. base.
The ground attack targeted the south side of FSB where the ground gently sloped away from the perimeter. The northeast side was marked by a steep slope toward a river, not advantageous terrain for a sapper attack. The sappers moved in small squads of three to six men, cutting four paths through the base's two outer concertina barriers. They took more time moving through the third barrier, which was about 20 meters from the bunker line, and then fanned out along the southwest side of the line. Following standard sapper doctrine, any attack would begin as soon as the mortar barrage commenced. The first 82 mm mortar shells hit FSB Mary Ann at 02:30, signaling the start of the ground attack.
Once through the wire, the sappers scattered toward key targets: the FSB's artillery, the bunker serving as the battalion tactical operations center (B-TOC), the company command post, and many of the perimeter bunkers. Their attack was aided by tear gas, delivered either by sappers (using grenades) or mixed in with regular high-explosive mortar rounds as part of the bombardment. Nolan also documents the VC use of CS gas in his narrative of the attack.
The attack against the B-TOC was made easier by Doyle's failure to post armed guards at the bunker's entrances (which was a violation of brigade policy), preventing any early warning. The sappers used a combination of CS gas and satchel charges in their attack on the bunker (coming at about 02:44 - the time is known because a radio operator in the bunker reported it to the FDC as direct mortar hits and the time was logged), effectively disrupting the command structure on the firebase. At about the same time, the radio operator requested illumination rounds from supporting artillery batteries, but did not indicate that Mary Ann was under ground attack. The south end of the B-TOC was burning by this time, the fire started by a satchel charge igniting a case of white phosphorus grenades located near the south bunker entrance.