just before the new year began, a specially-equipped american airplane traced Viet Cong radio signals to the hamlet of Tan Thoi in Dinh Tuong province, the province where the ARVN 7th Division was headquartered. Tan Thoi stood next to the hamlet of Bac, which would later be called Ap Bac after U. S. journalists covering the battle added the prefix ap (hamlet). Jutting abruptly upwards from flat rice paddies, the two hamlets stood out like small islands in a calm green sea. The Americans relayed the location of the Viet Cong radio transmitter to the South Vietnamese high command, which then ordered the 7th Division to take Tan Thoi at the beginning of January. Intelligence reports indicated that the Viet Cong forces guarding the radio transmitter consisted of a reinforced company of 120 men. Thus, the South Vietnamese leadership was aware that the infantry attack was likely to involve significant South Vietnamese casualties.

The Viet Cong actually had a total of between three and four hundred men at Bac and Tan Thoi, most of them belonging to the 261st and 514th Battalions. The 261st Battalion was among the best Viet Cong units in the country, a fact attributable to its fine leadership. Afterwards, American advisers were to say that the Viet Cong soldiers at Bac and Tan Thoi were the most determined Communist fighters they had encountered in more than one year.