[2]: 15 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.

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".

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.

[2]: 134–5 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.

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."

[4]: 342  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.

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.

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.

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.

[2]: 147 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.

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.

After calling for fire support, Doyle made the decision around 02:51 to evacuate the burning structure, and ordered the command staff to relocate to the aid station.

Prior to shifting the radios, Captain Paul Spilberg called for artillery fire "fifty meters out, three-hundred-and-sixty degrees around our position."

Once the radios had been established at the aid station, he and Doyle discovered that Company C's CP (the designated alternate location for the battalion command post) had been hit and partly destroyed.

Company C's 1st Platoon, occupying bunkers 15-19 along the side of the perimeter that had a steep slope, was relatively untouched by the initial assault and manned their positions in the trench line.

By contrast, 2nd Platoon in the southern sector had ten men killed and eleven wounded before Doyle and Spilberg moved toward the Company C command post.

The platoon leader, First Lieutenant Barry McGee, was killed fighting hand-to-hand with sappers, and the teams coming through that sector moved on to attack both the 155 mm howitzer position on the high ground to the northwest and the supply elements near the main resupply helipad.

Shortly after Doyle and Spilberg reached the partly destroyed Company C command post, the base's quad .50 began firing, "walking bursts down the hill and into the valley - and straight across into the next hillside".

[2]: 204–6 Although artillery had been firing in defense of Mary Ann since soon after the first word of the attack reached brigade headquarters, it was 03:25 before the first air assets appeared overhead.

Communications failures left both brigade and division headquarters believing that Mary Ann was only being shelled by mortars, making the need for air support appear less critical.

They provided what fire support they could using door guns and grenades, and before breaking station to refuel they landed at the small helipad and picked up "six or seven" of the most critical casualties and evacuated them to Chu Lai.

The North Vietnamese took the base under fire with a 12.7mm machine gun at about 16:00 that afternoon, wounding one American and reminding Mary Ann's garrison that they were still under observation.

Major Donald Potter, 1/46 Infantry's executive officer, issued instructions that the bodies be buried in an eroded section near the resupply LZ.

In July 1971, Baldwin was replaced as commander of the 23rd Infantry Division, with military sources quoted in news reports suggesting he was relieved because of the attack on FSB Mary Ann.

Historians taking this position include Shelby Stanton[4]: 342  and Lewis Sorley, giving the action high prominence in accounts of the U.S. Army's last years in Vietnam.

Sorley's account is particularly harsh, stating that the 1/46th Infantry "was riddled by drugs and incompetence" and that "[t]he disaster was compounded by a cover-up that extended all the way up to the Division commander.

"[7] Keith Nolan initially had a similar opinion, but later changed his mind after researching the action and writing his definitive account Sappers in the Wire.

Less than three years earlier, in August 1968, a Dac Cong sapper unit attacked the MACV-SOG Forward Operating Base 4 near Da Nang.