[1] However, with the Army of the Republic of Vietnam (ARVN) weakened by successive defeats at the hands of the communists, the South Vietnamese government faltering, and Saigon threatened with a major offensive, the worsening situation led to a significant escalation of the war in 1965, with a large-scale commitment of US ground troops under the command of General William Westmoreland.

[6] With the situation reaching crisis point during the Viet Cong wet season offensive in June 1965, Westmoreland requested further reinforcement and US and allied forces increased to 44 battalions which would be used to directly bolster the ARVN.

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, formed by the confluence of the Saigon and Thi Tinh rivers and Route 7, where they fought a number of significant actions including the Battle of Gang Toi on 8 November.

[15] During the period 21 November to 16 December 1 RAR was involved in Operation New Life in the La Nga Valley, 75 kilometres (47 mi) north-east of Bien Hoa in an attempt to deny the Viet Cong access to the rice harvest.

On 24 November D Company, 1 RAR carried out a deliberate attack on the fortified village of Duc Hanh which had been occupied by the Viet Cong, killing 10 and wounding four without loss.

[18][19] Located 4 kilometres (2.5 mi) west of the Iron Triangle, the Ho Bo Woods were believed to contain the political-military headquarters of the communist 4th Military Region, which controlled all Viet Cong activity around the South Vietnamese capital, Saigon.

Observing a lack of ground foliage, he was concerned about the possibility of extensive Viet Cong defensive works close to the LZ, and with the support of Williamson the landing zone was subsequently switched to a less-exposed location.

[36] No sooner had the lead elements—D Company under the command of Major Ian Fisher—emerged into the cleared area that was originally to have been used as the battalion's LZ, when the forward platoon came under fire from Viet Cong positions in the tree-line on the north-east corner.

[27] Converging on the suspected location of the communist headquarters, a thorough search of the area yielded little, and it was believed that the Viet Cong had withdrawn earlier in response to the initial Australian advance.

[41] Suspecting they were being drawn into a trap—as one of the American battalions of the brigade had been previously during Operation Hump[27]—the Australians moved into a tightly defended perimeter before dark and waited for the communists to counter-attack.

[43] The battalion spent a sleepless night and in the early hours of the morning there were a number of short exchanges of fire as small groups of Viet Cong returned to the area.

[35] Whereas standard US Army practice was to seal, blow up or otherwise attempt to render tunnel systems unusable with smoke, tear gas and explosives before quickly moving on, the Australians spent the next few days laboriously searching and mapping the complexes they found using military engineers.

[35] Led by MacGregor, the Australian sappers from 3 Field Troop systematically tackled the tunnels, using telephone line and compasses to plot the subterranean passages.

[48] In the southern area of operations, the US 3rd Infantry Brigade was making slow progress, with the Viet Cong using hit-and-run and ambush tactics to inflict casualties on the Americans, before withdrawing into the sanctuary of their tunnels.

[50] On 9 January, the Americans located and destroyed a cache of medical supplies, a small hospital and a large amount of rice, as well as capturing 30 Viet Cong during a series of skirmishes.

[34] To the north, while 1 RAR searched the tunnel complex the American battalions of the US 173rd Airborne Brigade continued to sweep their area of operations, with a number of friendly fire incidents complicating their task.

[51][52] Both the US 1/503rd and US 2/503rd Battalions searched eastward toward the Saigon River and, although they continued to find supply caches and abandoned positions, they were involved in only minor skirmishes with the Viet Cong.

[48][53] The Australians continued to explore the tunnels, finding a large quantity of documents and equipment, and by 10 January they had recovered 59 weapons, 20,000 rounds of ammunition, 100 fragmentation grenades, one 57 mm recoilless rifle, explosives, clothing and medical supplies.

[51] During the morning the battalion had conducted platoon-sized patrols south, south-west and west, locating a well-constructed defensive position, including bunkers and trenches connected by tunnels.

During this move the battalion, and armour from Troop D, US 16th Cavalry Regiment, engaged a dug-in Viet Cong Main Force company less than 2,000 metres (2,200 yd) west of the Australians.

[50] Advancing on a 1,500 metres (1,600 yd) front, troops from the 1st Battalion, US 28th Infantry Regiment—commanded by Lieutenant Colonel Robert Haldane—took a number of casualties from snipers, although the source of the fire was not immediately obvious.

[48] Led by Green, a squad of Americans equipped with flashlights, pistols and a field telephone penetrated over 1.6 kilometres (1 mi) of the tunnel system before becoming involved in a fire-fight with the Viet Cong.

[62] Yet even as the Americans were attempting to clear the tunnels, heavy hand-to-hand fighting broke out above ground and Haldane was later awarded the Silver Star for his actions when he rushed a bunker while under fire armed only with a pistol, in order to give first aid to a number of wounded soldiers.

[69] The same day, however, the Australian efforts to clear the tunnels had suffered a setback after an engineer, Corporal Robert "Bob" Bowtell, became stuck in a trap door between one underground gallery and another, more than 2 metres (6.6 ft) below the surface.

Despite the efforts of his comrades, he could not be recovered and died from asphyxiation after being overcome by a combination of tear gas, carbon monoxide, and lack of oxygen when he dislodged his respirator during the struggle to free himself.

[71] Meanwhile, the same day Haldane ordered US 1/28th Battalion to closely explore the tunnels in their area of operations, uncovering a number of chambers and trapdoors defended by grenades and booby traps.

[35] Although further American ground operations as well a number of heavy B-52 bombing raids resulted in further damage to the tunnel complexes, the Ho Bo Woods were never occupied on a permanent basis and the Viet Cong were successful in restoring their transit and supply functions.

[75] As such, despite significant disruption, the military and political apparatus in Cu Chi remained largely intact, allowing the communists freedom of action for later operations against Saigon.

[86] However, despite heavy casualties again being inflicted on the Viet Cong, the tunnels continued to remain a problem for the Americans and they were later used as a communist staging area for the attack on Saigon during the 1968 Tet offensive.

[87] Finally in 1970, American B-52 bombers carried out a number of heavy air-strikes on the area, dropping thousands of delayed-fuse bombs that buried deep into the ground before exploding, ending the tunnels' utility.