On previous occasions, US intelligence had tracked down the location of Viet Cong radio transmitters, but 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 carriers, and emergency replacements. Together, elements of the VC 261st and 514th Battalions in Ap Tan Thoi and Ap Bac formed a "composite battalion", which was placed under the command of Colonel Hai Hoang.