Initial Australian hypotheses

In the wake of the battle the Australians were left to speculate on the reason it occurred. One hypothesis was that the Viet Cong had intended to attack and overwhelm Nui Dat, with the initial plan to mortar the base to draw a response force into an ambush after which the base would be attacked and captured, but that they had been prevented from doing so after clashing with D Company. A second possibility was that they may have had the more limited aim of drawing D Company into an ambush to destroy it and secure a small victory over an isolated force. Finally, it was possible no ambush was planned at all, and that the Viet Cong had been moving on Nui Dat in regimental strength when they unexpectedly ran into D Company, resulting in an encounter battle.^[12][13][14] The evidence suggested they intended an attack on Nui Dat in some form, while the lack of prepared positions from which to mount an ambush made this unlikely.^[15] Brigadier David Jackson—Commander 1 ATF—believed the Viet Cong had been on their way to attack Nui Dat on the night of 18/19 August when D Company intercepted them and this was supported by information from a prisoner captured afterwards. They were carrying large quantities of ammunition and may have had sufficient strength to seriously damage the base, even if its defences were likely strong enough to withstand such an attack.^[16] Yet, from the earliest Jackson had felt the Viet Cong would have to engage 1 ATF in a major battle if they wished to retain control of Phuoc Tuy, and this interpretation fitted his previous thinking.^[17] In dispatching patrols east of Nui Dat following the mortar attack on 16/17 August Jackson seemingly reacted to his opponent's first move, and he may have allowed himself to be shaped into responding as the Viet Cong had intended.^[18]

In his after action report Lieutenant Colonel Colin Townsend—Commanding Officer 6 RAR—wrote that he believed D Company had disrupted a regimental attack on the 1 ATF base. Yet years later he stated that he doubted the Viet Cong ever intended attacking Nui Dat. He felt they would have gained more from attacking a soft target such as Ba Ria and that the defences at Nui Dat would have been sufficient to withstand a regimental assault, having been improved significantly in the 11 weeks since its occupation and protected by patrols and overwhelming artillery support, even if they were still to be fully developed.^[19] However, there were suspicions a battalion from the 274th Regiment was moving towards Nui Dat from the west at the same time the 275th Regiment approached from the east, and if true such an attack may have been feasible, even if it still would not have achieved the nine to one superiority communist doctrine required. The southern side of the base was only lightly defended and an assault from that direction would have allowed the Viet Cong to attack the gunline first, and if they succeeded in breaking-in would have then crossed 500 metres (550 yd) of open ground before reaching Headquarters 1 ATF.^[19] On the other hand, Major Noel Ford—Officer Commanding B Company, 6 RAR—believed the mortar attack served to lure an Australian reaction force into an ambush, arguing the Viet Cong would have been unlikely to telegraph their intentions to attack Nui Dat and that they had left a clear trail for any force dispatched to find the mortar base plate and RCL positions. Townsend disagreed, arguing the mortar crews and RCL teams had actually tried to hamper the follow-up by breaking into small groups, indicating that no such trap had been planned.^[20] Meanwhile, Major Harry Honnor—Officer Commanding the 161st Battery, Royal New Zealand Artillery attached to 6 RAR in direct support—believed the bombardment had been used by the Viet Cong to register targets and that D Company had stumbled across the force as it was preparing to launch an assault on Nui Dat two days later, with the battle that ensued preventing this from occurring.^[21]

A year after, Nguyen Van Nuong (alias Loc), a former Viet Cong soldier who had rallied, claimed to have been second-in-command of a platoon from D445 Battalion at Long Tan. During questioning he stated they had planned to destroy Nui Dat to allow further operations in Phuoc Tuy, with the force consisting of three battalions from the 275th Regiment, D445 Battalion and several medical units; in total some 2,000 men. The Viet Cong had estimated Australian strength at Nui Dat at 3,000 men and planned to mortar the base to draw a response force into an ambush by two battalions from the 275th Regiment and D445 Battalion, while the remaining battalion would capture Nui Dat. They expected to quickly overwhelm the ambushed force and any relief force, thereby leaving the base relatively undefended. Loc said D445 Battalion's role was to close the rear of the ambush and that they taken only light casualties during the battle. Despite Viet Cong claims he believed only 100 Australians had actually been killed or wounded, and that the official number of Viet Cong casualties was 200 dead, although he had seen more than that on the battlefield and believed many more had died during the fighting. According to Loc the Viet Cong were concerned about 1 ATF's capabilities and would attempt to avoid contact in future.^[22] In accounting for the reason Nui Dat ultimately escaped attack it was surmised that D Company may have entered the killing zone before the ambush was prepared and because of this the third battalion may have been unable to be released. Other possibilities included the Viet Cong being forced to abandon that part of the plan by the weight of the artillery, or that they had overestimated the size of the Australian force and retained the third battalion to complete the ambush.^[23]

The information from Loc was consistent with that known or suspected by 1 ATF, and other than a single battalion overwhelming Nui Dat, was considered plausible. Yet Smith was adamant D Company had not been ambushed, believing the action to have been an encounter battle regardless of what may have been planned. He argued that the troops in the rubber plantation were not sited for an ambush, while the presence of 2,000 to 3,000 men so close to Nui Dat without prepared positions indicated they were still moving when the first contact occurred.^[23] Equally, D Company had been left in possession of the dominant terrain and was not drawn into a killing zone as the plantation offered too much concealment, no effort had been made to cut them off from any relief force sent from Nui Dat, no artillery or mortars were fired into the engagement area, while the thick vegetation to the north prevented the Viet Cong manoeuvring into a favourable position. He reasoned that an ambush would have been more effective near the original base plate position as that was the only location they could have been sure the Australians would investigate given they had no way of knowing D Company would move another 1,500 metres (1,600 yd) east after the handover with B Company where they contacted the 275th Regiment. Finally, communist ambushes were doctrinally completed in less than three hours, but Long Tan had continued for over four. As such Smith concluded that since the tactics used by the Viet Cong had been contrary to many of the principles of an area ambush the battle was unlikely to have been one.^[23] McNeill argues though that too many facts may be missing to make a conclusive assessment of Viet Cong intentions as to date no definitive Vietnamese account is available, while according to Ham those that exist are contradictory or unreliable.^[24][25]

Later Vietnamese claims

In 1988 two former PAVN officers, Nguyen Thanh Hong and Nguyen Van Kiem (a later commander of D445 Battalion)—while being interviewed by the Australian official historian, Ian McNeill—claimed no more than two battalions had been involved, less than half the lowest Australian estimate. Kiem claimed the primary force consisted of D445 Battalion reinforced by one company of North Vietnamese from Headquarters Military Region 7 with a total of 720 men, while Hong said one battalion from the 275th Regiment had also been involved and that with D445 Battalion its total strength was 700 to 800 men. They believed an entire Australian battalion had been engaged along with two squadrons of APCs, and that it had been reinforced by a second battalion the following day, while there had been hundreds of Australian casualties, citing a BBC broadcast which reported 500 killed and 21 tanks destroyed. Against this, Kiem estimated total Viet Cong losses as approximately only 30, mainly from artillery fire. Hong agreed, stating the figure of 245 killed was exaggerated by a factor of 10.^{[26][Note 1]} Estimated losses from D445 district-commander Colonel Bao stated information he had received at the time was that there was 150 losses including killed and captured for all units involved.^[8] Contradictory evidence is also given by the D-Company Commander, who stated he "didn't seen more than 50 bodies and I spent three days in the area."^[8]

Yet this was at odds with alleged Viet Cong records to have been captured by US forces of the diary of the 275th Regiment's commander listing that unit's losses as 500 dead.^[28] McNeill provides these figures based on the diaries of a future Deputy Defense Minister Nguyen Thoi Bung and overall battle commander; however, according to Earnest Chamberlain actual documentation of these diaries have not been corroborated.^[29] Captured soldiers later stated D445 Battalion's casualties had been 70 killed and 100 wounded.^{[30][Note 2]} These claims were directly contradicted by Australian intelligence reports in which the D245 Unit suffered just 30 casualties.^[32] Claims were also contradicted by the Dong Nai history which stated both the 275th Regiment and D445 Battalion had been involved.^[33] Indeed, the majority of the dead buried after the battle were wearing green uniforms, and this coupled with their weapons and equipment, indicated they were either main force Viet Cong or North Vietnamese.^[34][35]

One of the three prisoners was from D445 Battalion and the other two were North Vietnamese who claimed to be from the 45th Regiment, perhaps unaware their unit had changed its name to the 275th Regiment when operating in the south. According to them it was the artillery battalion of this regiment which had bombarded Nui Dat.^[36] Years later many of Kiem's claims about the role of D445 Battalion were found to be misleading.^{[37][Note 3]}

The Long Tan Cross, commemorating Australian losses

Both Kiem and Hong claimed the plan had been to mortar Nui Dat to draw a reaction force into an ambush, not to attack the base. They confirmed the dual military and political aim, emphasising the disruption caused by 1 ATF lodging at Nui Dat and the intention to affect Australian public opinion.^[38] The forced evacuation of Long Tan and Long Phuoc had curtailed the Viet Cong who regarded it as a liberated area, and it was chosen as the ambush site to demonstrate to the local population that they were still able to operate despite the Australian presence. Hong explained the ambush was sprung within range of the artillery at Nui Dat because they correctly assumed no reaction force would move outside the cover it provided, and instead planned on rapidly closing with and destroying it before artillery or air support could be brought in. They planned to occupy prepared positions in the rubber plantation and destroy whatever force was sent to locate the mortars following the attack on Nui Dat. A stop force had been placed to the east, while another was to the south to hold the Australians in place, reasoning that the thick undergrowth to the north would force them between the two. D445 Battalion was then to close the rear of the ambush, attacking from the north-west. A larger ambush would then have been staged outside artillery range along the withdrawal route subsequently used by the 275th Regiment. Meanwhile, the 274th Regiment was positioned north of Binh Ba astride Route 2 to ambush US forces should they try to relieve 1 ATF.^{[39][Note 4]} Hong later asked: "How can you claim a victory when you allowed yourselves to walk into a trap that we had set? Admittedly we did not finish the job, but that was only because time beat us and your reinforcements arrived. I mean you did not even attempt to follow us up. How can you claim a significant victory from that sort of behaviour?"^[3]

While the plan outlined by Hong had many of the components of an annihilation ambush, and drawing the enemy out his base area was a standard communist tactic, several weaknesses were evident, including the lack of a means of neutralising the Australian artillery, the absence of indirect fire weapons in the ambush, and the comparatively small size of the ambushing force. Equally the plan to site a second ambush outside artillery range would have been ineffective as the Australians were unlikely to proceed that far unless their guns had followed. Hong believed the ambush failed because the soldiers at the front fired on 11 Platoon before the entire Australian force entered the killing zone, while it should have been opened from the rear against 12 Platoon. Had D Company been attacked from the rear the battle would have been more like an ambush, and they would have been in immediate danger, while it would likely have been irrelevant if the forward Viet Cong troops fired first as the Australians were already well into the plantation.^[41] Yet they had been unable to co-ordinate such a manoeuvre and D445 Battalion had more than three hours from the initial contact to close the gap at the rear and failed to do so. Regardless, a few issues cast doubt on this version of events. Firstly, if the Viet Cong had been in the plantation on 17 and 18 August preparing an ambush they would likely have been identified by the patrols from A or B Company, 6 RAR. Secondly, the alleged size of the force is questionable as just 700 men would have been insufficient to be certain of success, while according to communist doctrine it would have required more than a regiment to achieve overwhelming force. In the end the reaction of the Viet Cong indicted they had not detected D Company's presence in the rubber plantation until the first clash with 11 Platoon.^[42]

In 2005, Nguyen Nam Hung—deputy commander 274th Regiment—claimed the Viet Cong had indeed intended to destroy 1 ATF and liberate Ba Ria province. He stated that a two-phased offensive had been conceived, planning first to draw them into the open and destroy them in an ambush, and then attack the weakened base at Nui Dat. Ultimately though their goal had been political and the capture of Nui Dat was not considered important. According to the official D445 Battalion history, the officers of the units involved met in early August to finalise plans for the attack.^[43] The account claimed that following the mortar attack on 16/17 August, the force had formed up in a 3-kilometre (1.9 mi) wide semi-circle at 10:15 on 18 August to lie in wait, with the 275th Regiment given the "middle battle" position and tasked with springing the ambush, while other units from the 275th Regiment and one company from D445 Battalion occupied the "front block" position near the village of Long Tan, and two companies from D445 Battalion under command of Major Sau Thu had been tasked with occupying the "back block" position to encircle the Australians by sweeping beneath their "tail" and destroying them. In support, the 80-strong Vo Thi Sau militia company, most of them women, was located nearby to help drag away the wounded following the battle.^[25] Thu later confirmed he had been ordered to lure the Australians out of Nui Dat, kill as many as possible, capture their weapons, and overrun the base.^[44] Yet this explanation also contains inconsistencies. In particular, the delay of over a day between the mortar attack and the battle suggests another purpose, and it is possible it may have served to test the Australian reaction and the speed with which counter-battery fire could be directed against them in preparation for a full-scale attack.^[25]