with eavesdropping equipment located a VC radio transmitter. It intercepted radio signals in the hamlet of Ap Tan Thoi in Dinh Tuong Province, where the ARVN 7th Infantry Division was headquartered. The radio intercept and other information obtained by Jim Drummond, Vann's intelligence officer, indicated that the VC used Ap Tan Thoi as a headquarters location. Furthermore, South Vietnamese and American intelligence personnel believed the VC had deployed a reinforced company of about 120 men to protect the transmitter. Confident that the VC unit was no larger than the reported number, the ARVN 7th Infantry Division was instructed to attack Ap Tan Thoi. An operations plan suited for an attack on a small enemy formation was drafted by Ziegler, an adviser to Dam and the command staff of the 7th Infantry Division. Ziegler's plan, codenamed Operation Duc Thang I, called for the South Vietnamese to assault Ap Tan Thoi from three different directions: three rifle companies from the 11th Infantry Regiment, 7th Infantry Division, to move from the north; the Dinh Tuong Civil Guards Regiment to march north from the south in separate columns; and a company of 13 M113 armored personnel carriers with an infantry company on board from the southwest. The M113 carriers and the infantry company could act as both a mobile reserve and a reaction force, so it was positioned where it could be shifted to the contact area if the VC began to retreat. In addition, Dam would also deploy two rifle companies at Tan Hiep airfield (), which could be brought to the battlefield by helicopters from the US Army 93rd Transportation Company. On previous occasions, US intelligence had tracked down the location of Viet Cong radio transmitters. However, those were often relocated before the South Vietnamese launched their attacks, so Ziegler privately questioned if the VC had as many as 120 soldiers in Ap Tan Thoi. However, in 1963, the VC had changed its policy from avoiding the ARVN to standing and fighting. The 1st Company, 261st Battalion, and the 1st Company, 514th Battalion, had a total strength of 320 regular soldiers and were positioned in Ap Bac and Ap Tan Thoi, respectively, which were separated by a distance of about . The combined companies were supported by approximately 30 local force soldiers from Chau Thanh District who served as scouts, ammunition bearers, litter-bearers, and emergency replacements. Together, elements of the VC 261st