destroy operations using the newly developed doctrine of airmobile operations, utilising helicopters to insert light infantry and artillery into an area of operations, and to support them with aerial mobility, fire support, casualty evacuation, and resupply. The battalion commenced operations in late June 1965 and initially focussed on defeating the Viet Cong's wet season offensive. During this time US 173rd Brigade, including 1 RAR, conducted a number of operations into War Zone D—a major communist base area at the junction of Phuoc Long, Long Khanh, Bien Hoa and Binh Duong provinces—as well as in the Iron Triangle in November. Prelude Opposing forces By late-1965 the junction of the Song Be and Song Dong Nai rivers had become a major communist staging area for men, equipment, and supplies for units based around Saigon and the Mekong Delta. Communist communications and resupply routes between War Zone C and D also met in this area. Westmoreland planned to use the US 173rd Airborne Brigade to keep the Viet Cong off balance and to target their base areas, and consequently a search-and-destroy operation codenamed "Operation Hump" was planned. Operation Hump marked the half-way point in the twelve-month tour of duty of the 173rd Airborne Brigade and was named accordingly. The concept of operations envisioned 1 RAR and the US 1st Battalion, 503rd Infantry Regiment being inserted by helicopter during an airmobile operation into War Zone D, in an area about north-east of the Bien Hoa Air Base. The Australian and American area of operations (AO) was to be separated by the Song Dong Nai, with 1 RAR to deploy into a landing zone (LZ) to the south, while the 1/503rd would conduct a helicopter assault onto a LZ northwest of the Song Dong Nai and Song Be. Meanwhile, the 173rd Airborne Brigade's second battalion—2nd Battalion, 503rd Infantry Regiment—would be left to defend Bien Hoa Air Base. The area was thought to be a Viet Cong stronghold and American intelligence initially identified Q762 Main Force Regiment and D800 Main Force Battalion as being nearby. However, unknown to the allied force, the Viet Cong 9th Division had likely received forewarning of the operation and had deployed one of its most experienced regiments, supported by a number of local force battalions, determined for a test of allied strength. The 271 Main Force Regiment (also known as Q761 Main Force Regiment) subsequently took up defensive positions in the