'Ri' was the Japanese seizure of Buna and Gona as beach-heads on the north-east coast of Papua as the starting points for an overland offensive along the Kokoda Trail (otherwise the Kokoda Track) against Port Moresby on the south coast of what was then the Australian Territory of New Guinea (21/29 July 1942).

Shortly before this time the Allies had planned to build three airfields from which the proposed campaign to the north-west along the coast of Papua and North-East New Guinea could be supported: the Australians landed a small force at Milne Bay on 25 June, and within a fortnight had also shipped in Brigadier John Field’s Australian 7^th Brigade to serve as the basis of Major General Cyril A. Clowes’s Milne Force for the protection of the area; the small Australian force at Wau, named as 'Kanga' Force and commanded by Major Norman Fleahy, was reinforced and moved against the Japanese forces at Salamaua and Lae, but was driven back; and it was planned that on 10 August 'Providence' would land a US Army engineer unit at Buna to start work on an airfield there.

However, 'Providence' was forestalled by 'Ri', which was planned and undertaken by Admiral Shigeyoshi Inoue’s 4^th Fleet. The campaign resulting from 'Ri' took the form of a series of battles fought from July 1942 ultimately to January 1943 between Japanese and Allied (primarily Australian) forces. The 'Ri' plan was the responsibility of Lieutenant General Harukichi Hyakutake’s 17^th Army, formed on 18 May 1942 after the end of Japanese operations in the Netherlands East Indies and on Bataan on the island of Luzon of the Philippine islands group, to control the Japanese land operations in New Guinea and in the Solomon islands group with the primary objects of capturing New Caledonia, Fiji, Samoa and Port Moresby. In this way Allied, the Japanese planned, maritime and air communications between the USA and Australasia would be severed, and the Allies deprived of the southern base area for any northward advance toward the Philippine islands group. A Japanese base at Port Moresby would also allow Japanese air and sea power to dominate eastern Australia, and allow the mounting of an invasion of Australia should such an undertaking be planned.

The 17^th Army succeeded, and indeed absorbed, Major General Tomitaro Horii’s South Seas Detachment, and during June began to form at Truk atoll in the Caroline islands group, in the Palau islands group and, nearest New Guinea, at Rabaul on New Britain, with Major General Yumio Nasu’s 'Aoba' Detachment, Major General Kiyotake Kawaguchi’s 'Kawaguchi' Detachment and Colonel Kiyomi Yazawa’s 'Yazawa' Detachment made available by the end of earlier operations.

The related 'Mo' (ii) and 'Mi' (ii) operations, against Port Moresby and Midway island respectively, were just beginning at this time, and all looked promising for the joint offensive into the South Pacific by the Japanese army and Japanese navy. Then the Battles of the Coral Sea and Midway stopped both of these Japanese operations in their tracks, and 'Mo' (ii) had to be rethought as an overland advance on Port Moresby, while the plans for the 'Fs' seaborne assaults on New Caledonia, Fiji and Samoa were temporarily cancelled.

The operation in New Guinea was recast as an advance across the Owen Stanley mountain range from a consolidated beach-head at Buna and Gona on the north-east coast of Papua. For this operation Hyakutake assigned the South Seas Detachment reinforced with the 15^th Independent Engineer Regiment, the latter added as intelligence reports had indicated that the Kokoda Trail was passable, but the Japanese now hoped to be able to develop it into a road capable of taking trucks, or at worst pack animals.

Lae and Salamaua were already in Japanese hands and being used as advanced air bases for the bombing of Port Moresby, but could be nothing more than intermediate steps on the route to Buna and Gona. Hyakutake could achieve little without the navy, and such support was initially supplied by Inoue’s 4^th Fleet at Rabaul. This formation was replaced on 18 July by Vice Admiral Gunichi Mikawa’s 8^th Fleet, which initially possessed five heavy cruisers, three light cruisers, five submarines, several destroyers, and Rear Admiral Sadayoshi Yamada’s 25^th Air Flotilla. With Mikawa’s assistance, Hyakutake planned to secure his beach-head with the 1,800 men of Colonel Yosuke Yokoyama’s 'Yokoyama' Force, which comprised the 1/144^th Regiment, one naval infantry company of the 5^th Sasebo Special Naval Landing Force, Yokoyama’s own 15^th Independent Engineer Regiment and a number of artillery, anti-aircraft and service units.

The 'Yokoyama' Force embarked during 18 July in three high-speed transports (5,973-ton Ryuyo Maru, 9,788-ton Ayatozan Maru and 9,310-ton Kinryu Maru), and was escorted from Rabaul by Rear Admiral Koji Matsuyama’s 18^th Cruiser Division (light cruisers Tatsuta and Tenryu), minelayer Tsugaru, and the 29^th Destroyer Division comprising Asanagi, Ukuzi and Yuzuki, a number of lighter vessels including the submarine chasers Ch-28, Ch-29, Ch-30 and Ch-32 as well as some minesweepers. As the forces boarded, Mikawa was completing the plans for the complementary 'Re' to establish a seaplane base in the Louisiade islands group off Milne Bay, later changed to a landing in this bay to take the three airstrips being built there by the Australians.

The 'Ri' convoy departed Rabaul on 20 July and reached Gona on the following day without encountering any air or surface opposition, and the troops began to land during the afternoon of the same day against no ground opposition. A few Allied aircraft appeared later in the day and damaged two of the transports, but all except 40 of the soldiers got ashore safely.

One platoon of the Papuan Battalion, made up of indigenous Papuan soldiers under an Australian officer, Lieutenant John Chalk, was the only Allied unit in the area, and reported the arrival of the Japanese on 22 July. During the following night Chalk and his 40-strong unit ambushed Japanese forces from a hill overlooking the road linking Gona and Sangara road, and then pulled back into the jungle. On 23 July, the area was attacked by Boeing B-17 Flying Fortress heavy bombers and Martin B-26 Marauder medium bombers of the USAAF: Ayatosan Maru was sunk and Uzuki was slightly damaged. Daily raids against the area of Buna and Gona were then launched by warplanes of the USAAF and the Royal Australian Air Force. Further Japanese attempts to increase their strength at Buna managed to get past the Allied air forces. A transport convoy arrived on 25 July, but the 6,701-ton transport Kotoku Maru was sunk on 29 July, though most of the embarked troops managed to get ashore. A third transport was forced to return to Rabaul, and a whole convoy (the minelayer Tsugaru and a number of submarine chasers escorting one freighter) was forced to turn back on 31 July. Finally, a convoy of three freighters, escorted by Tatsuta, Uzuki and Yuzuki, departed Rabaul on 5 August but was ordered to turn back two days later after the Japanese had started to digest the implications of the US 'Watchtower' landing on Guadalcanal to the south-east in the Solomon islands group. Even so, the Japanese had managed to capture the area of Buna and Gona and been able to reinforce their advance party, albeit not as rapidly as they had hoped. Further reinforcements in a convoy under Mikawa managed to get through on 14 August and land some 3,000 Japanese, Korean and Formosan troops of the 14^th Naval Construction Unit and 15^th Naval Construction Unit. On 17 August there arrived the 5^th Sasebo Special Naval Landing Force, and the rest of the 144^th Regiment under Lieutenant Colonel Hatsuo Tsukamoto, 55^th Mountain Artillery Regiment, 47^th Anti-Aircraft Artillery Regiment and 55^th Cavalry Regiment, and on 21 August two battalions of the 41^st Regiment were delivered by sea.

Yokoyama ordered Tsukamoto to advance overland and capture the airstrip at Kokoda, and also to undertake a reconnaissance in force along the Kokoda Trail. These forces encountered the Australian troops deployed near Kokoda in the northern foothills of the Owen Stanley mountains, thereby initiating the Kokoda Trail campaign.

The Kokoda Trail is single-file track extending some 40 to 65 miles (65 to 105 km), depending on definition, between a point just outside Port Moresby on the Coral Sea, across the mountains of the Owen Stanley range, to Kokoda and thence to the coastal lowlands adjacent to the Solomon Sea. The track crosses some of the most rugged and isolated terrain in the world, reaches an altitude of some 7,380 ft (2250 m) at Mt Bellamy, and is generally characterised by hot, extremely humid days and cold nights, torrential rain, and endemic tropical diseases. The track is passable only on foot, a fact which had considerable effects on the logistics of the campaign as well as the forces which could be used and the type of warfare in which they could engage.

(Before World War II, paths in remote areas of New Guinea were commonly known as tracks, but the appellation Kokoda Trail rather than Kokoda Track was made more familiar by US reporters.)

General Douglas MacArthur, heading the South-West Pacific Area command, had been considering the ways in which his forces could counter the Japanese advance into the South-West and South Pacific Areas, and decided to enlarge the Allied forces in New Guinea as an essential first step in preparing an offensive against the main Japanese bases at Rabaul on New Britain and Kavieng on New Ireland. Lieutenant General Sydney F. Rowell, commanding the Allies' New Guinea Force, the operational command which had replaced Major General Basil M. Moorhouse’s 8^th Military District administrative headquarters in Port Moresby, had ordered the 100-strong B Company of the 39^th Battalion, an Australian militia unit, to move along the Kokoda Trail over the mountains of the Owen Stanley range to the village of Kokoda, where it was to secure the airstrip in anticipation of an Allied build-up along the north coast of Papua.

While the militia company was securing its positions, as far forward at Wasida on the eastern side of the Kumusi river, news reached it of the 'Ri' landings on the north-east coast in July 1942, and the fact that the landed troops had swiftly set about the task of establishing small but significant beach-heads at Buna, Gona and Sanananda.

Encountering the Australian troops deployed forward of Kokoda on 23 July, Tsukamoto deployed his infantry and naval troops and, after some skirmishing and several Japanese assaults, the outnumbered Australians fell back to Kokoda, where the Japanese attacked them on 28 July. The Australians soon retook Kokoda, but after two days a Japanese attack forced the Australians into another retreat, and on 29 July the Japanese took Kokoda. The defence was outnumbered, and lacked both heavier weapons and adequate supplies of food, but fought so well that the Japanese believed they had overcome a force of more than 6,000 men.

Having established the defensive capability of the Australians and with the vital supply base and airstrip at Kokoda within his grasp, Tsukamoto deemed the track to provide practicable access for a full-scale overland assault against Port Moresby. Forward-based at Rabaul, Horii’s 10,000-man South Seas Detachment was now allocated the task of taking Port Moresby. Horii’s force landed at the Papuan beach-heads and began moving up the Kokoda Trail.

The loss of the Kokoda airstrip compelled the Australians to send the other companies of the 39^th Battalion, as well as the 49^th and 55^th Battalions constituting the rest of Brigadier Selwyn H. W. C. Porter’s 30^th Brigade, forward along the Kokoda Trail rather than by air. The associated supplies, which had previously been flown to Kokoda by Douglas C-47 Skytrain transports of the USAAF, had now to be carried by Papuan porters, and the same means had to be used, in the other direction, for the evacuation of the wounded and sick. Belatedly, MacArthur and the Allied land forces in Papua, General Sir Thomas Blamey, who was also commander-in-chief of the Australian Military Force, realised how serious the situation had now become and ordered Major General Arthur S. Allen’s 7^th Division, recently arrived back in Australia from the Middle East, to embark for New Guinea. Brigadier Arnold W. Potts’s 21^st Brigade was the first element of the division to reach Port Moresby. The brigade comprised the 2/14^th, 2/16^th and 2/27^th Battalions. The 2/14^th and 2/16^th Battalions immediately began to move to the north along the Kokoda Trail to reinforce the Australian detachment outside Kokoda, now known as the 'Maroubra' Force within Rowell’s New Guinea Force. The 2/27^th Battalion had also been allocated to the Kokoda Trail operation, but following the Japanese 'Re' landings at Milne Bay off to the east, was instead held in Port Moresby as the divisional reserve.

The first reinforcements from the 30^th Brigade, led by the brigade major, Major Cameron, reached the Australian forces outside Kokoda, and Cameron assumed local command pending the arrival of Lieutenant Colonel H. Ralph Honner, the 39^th Battalion’s commanding officer. Cameron led an abortive attempt to retake Kokoda before being forced to withdraw. On taking command, Honner ordered a withdrawal farther to the south along the Kokoda Trail to Isurava.

Horii moved the 2,500-man vanguard of his South Seas Detachment against the 39^th Battalion and elements of the 49^th and 53^rd Battalions, a strength of about 400 men. The Japanese force made contact with the outer positions of the 'Maroubra' Force and began frontal attacks against the dug-in defenders with the aid of a mountain gun and mortars which had been manhandled up the Kokoda Trail from the coast. Japanese reconnaissance had revealed a parallel track bypassing Isurava, defended by the 53^rd Battalion, and one Japanese battalion detached to force this route met with success as the demoralised 53^rd Battalion gave ground, retreating to the junction with the Kokoda Trail behind Isurava. Several of the 53^rd Battalion’s more senior officers were killed, leading to the battalion’s further demoralisation.

During the battle, the first troops of the 21^st Brigade arrived to reinforce the 39^th Battalion. Potts assumed command of the 'Maroubra' Force, and using the screen provided by the 39^th Battalion, deployed the 2/14^th Battalion at Isurava and sent the 2/16^th Battalion to take over defence of the alternate track from the retreating 53^rd Battalion. By the time the 2/14^th Battalion had deployed, the Japanese were able to field a strength of some 5,000 men and thus outnumbered the Australians by at least five to one.

The tactics used by the Japanese were essentially the same as those which had proved so effective in the 'E' (i) Malaya campaign of late 1941 and early 1942: pin the opposing force with a frontal attack regardless of its cost while feeling out any exposed flank with a view to bypassing the opponent’s main strength and then cutting his lines of communication before falling on his rear.

Horii was operating against the clock imposed by the Papuan terrain, season and conditions: any delay exacerbated the existing debilitation of the Japanese force from disease and lack of food. As a result, the 'Maroubra' Force endured four days of violent frontal attacks. During the fighting, the 39^th Battalion was compelled to remain in the line instead of being relieved, as the Japanese threatened several times to break through the perimeter of the 2/14^th Battalion. Australian casualties mounted steadily, however, and ammunition ran low. The Japanese threatened to make a breakthrough on the alternate track, and Horii had now deployed several companies on the flanks and near the rear of the 2/14^th and 39^th Battalions, threatening an encirclement.

Outnumbered and now nearly outmanoeuvred, the 'Maroubra' Force pulled back toward Nauro and Menari. Potts relieved the exhausted 39^th Battalion and the shattered 49^th and 53^rd Battalions, the malaria-stricken survivors making their way back to Port Moresby on foot or on litters carried by Papuan bearers. Retreating soldiers, Papuan porters and wounded men immediately flooded the Kokoda Trail, and as a result the trail rapidly became a sea of mud in some places. There was no terrain suitable for the creation of a defensive position between Isurava and a feature known as Mission Ridge, lying to the south of Nauro and Myola. As a result, Potts and the 'Maroubra' Force retreated through Menari, mounting small delaying actions wherever and whenever possible.

Up to this time Myola had been a supply dump, a cleared patch of ground allowing supply drops by the USAAF’s C-47 'biscuit bombers'. Allen asked Potts when offensive action would be resumed, now that air drops were ensuring a regular if sparse and intermittent flow of supplies. Potts in turn enquired after the availability of his 2/27^th Battalion, denied him by Allen in view of the situation at Milne Bay. Pressured by Rowell and Blamey, Allen ordered Potts to hold Myola as a forward supply base, and to gather sufficient supplies for a riposte against the Japanese advance. The orders revealed that there was an almost total ignorance among the commanders and staffs in Port Moresby of the real nature of the terrain, conditions and opposition faced by the 'Maroubra' Force on the western downslopes of the Owen Stanley range, but Potts knew only too well the overwhelming superiority of numbers fielded against him.

Threatened with an outflanking manoeuvre along a loop of the Kokoda Trail, and lacking defensible terrain near Myola, Potts was forced to retreat through this place, destroying the supply base behind him. The 'Maroubra' Force thus withdrew to the next defensible strongpoint on the Kokoda Trail, a feature known as Mission Ridge.

With the 'Re' operation against Milne Bay halted, Allen finally released the 2/27^th Battalion from the divisional reserve at Port Moresby. After moving up the Kokoda Trail from Port Moresby, the fresh battalion finally joined the 'Maroubra' Force at Mission Ridge, and Potts was thus able to commit his entire brigade, or rather what was left of the brigade, to the battle. Taking up positions on a hilltop straddling the trail, which later became known as 'Brigade Hill', the 'Maroubra' Force awaited the Japanese advance. The usual Japanese frontal attacks soon began to materialise against the leading elements of the Australian defence. The Japanese also launched a strong flank attack, however, with the object of cutting off the forward elements from the rest of the 'Maroubra' Force. The flank attack succeeded in cutting the 'Maroubra' Force in two, and thus separated the brigade headquarters from its three battalions. With brigade headquarters on the verge of being overrun, Potts and the rear elements of the 'Maroubra' Force were forced to retreat along the Kokoda Trail to the village of Menari.

When it became clear that they were in danger of being cut off and destroyed, the remaining soldiers of all three Australian battalions immediately left the Kokoda Trail and 'went bush' via an alternate track back to Menari. The 2/14^th and 2/16^th Battalions managed to rejoin Potts and the 21^st Brigade headquarters at Menari, but the 2/27^th Battalion was unable to reach the village before the rest of the brigade was again forced to retreat by the continued Japanese advance. The 2/27^th Battalion, along with wounded from the other battalions, were forced to travel by paths paralleling the main Kokoda Trail, eventually making their way back to Ioribaiwa, and thence to Imita Ridge. Elements of the 2/14^th and 2/16^th Battalions accompanying Potts later managed to regroup for the defence of Imita Ridge, but the 2/27^th Battalion managed to regroup only much later, after the start of the Japanese retreat.

Controversy surrounds some of reports of the actions by the three Australian battalions and their commanders, but the result of the action was the shattering of the 'Maroubra' Force. The defeat of the 21^st Brigade at 'Brigade Hill' finally ended the defensive capability of the 'Maroubra' Force as a cohesive unit, and was a decisive victory for the Japanese. Blamey ordered Potts to report to Port Moresby immediately 'for consultations', replacing him as commander of the 'Maroubra' Force with Porter.

On reaching Ioribaiwa, the leading Japanese elements began to celebrate as, from their vantage point on the hills around Ioribaiwa, they could see the lights of Port Moresby with the Coral Sea beyond them. The Japanese were ordered to dig in on the ridge, however, for by this time it had become clear to Horii that the Japanese lines of communication along the Kokoda Trail from Buna were on the verge of total collapse. No supplies had reached the forward Japanese battalions for some days now, and the few meagre supplies captured from the Australians were insufficient to feed any resumption of the offensive. The food found in the former Australian supply dump at Myola had proved to be contaminated, and as a result hundreds of Japanese soldiers were now succumbing to dysentery, while others were showing the advanced stages of starvation.

Meanwhile, the totally exhausted men of the 'Maroubra' Force were relieved by Brigadier Kenneth W. Eather’s 25^th Brigade and Brigadier John E. Lloyd’s 16^th Brigade of Major General Allan S. Boase’s 6^th Division. The newly arrived Australian brigades dug in at Imita Ridge, near the start of the Kokoda Trail outside Port Moresby, and were supported by a battery of 25-pdr field gun/howitzers, which had been dragged up the Kokoda Trail.

At this time, Horii received orders from Rabaul: as a result of the continuing Japanese commitment to the Battle of Guadalcanal, no more reinforcements could be spared for the Kokoda Trail offensive and Horii was therefore to pull back to the beach-heads at Buna and Gona.

The Japanese began rapidly to move back toward Kokoda. With two Australian brigades committed to action on the Kokoda Trail, Allen now assumed operational command in the area. Each brigade in turn kept contact with the withdrawing Japanese until resistance began, just to the south of Kokoda, and later to the west. Unsatisfied with the speed of his advance, Blamey relieved Allen of command and replaced him on 14 September with Major General George A. Vasey.

Several grisly discoveries by advancing Australian troops starkly illustrated the logistical nightmare attendant on operations along the Kokoda Trail: Japanese corpses were often found with no sign of external trauma, having died from typhoid and dysentery, and several corpses of Australian soldiers were found to have had body parts removed, a result of the starving Japanese resorting to cannibalism.

In an attempt to cut off the Japanese at the Kumusi river crossings, the 126^th Infantry of Major General Edwin F. Harding’s US 32^nd Division set off on an advance from Port Moresby along tracks parallel to the Kokoda Trail. However, the Japanese withdrawal was more rapid than expected, and the 126^th Infantry finally emerged near the Gona and Buna beach-heads without encountering the Japanese. Tropical diseases and exhaustion took their toll on the 126^th Infantry, which had thus lost a significant part of its strength even before the start of the subsequent Battle of Buna and Gona.

In a dramatic and bizarre turn of events, Horii disappeared, presumed drowned, while withdrawing with his troops across the Kumusi river toward the beach-heads: the strong current swept away a horse on which he was riding, and Horii decided to float down the Kumusi river in a canoe with other senior officers, in order to get back to Buna as quickly as possible and start the preparation of the beach-head defences. The canoe was floated down to the river mouth, but Horii and his staff were swept out to sea in a freak squall and none of the men was ever seen again.

After the fighting, withdrawal and the relief of the 21^st Brigade by the 25^th Brigade, Blamey visited the remnants of the 'Maroubra' Force near base camp, outside Port Moresby. He relieved Potts of his command, citing Potts’s failure to hold back the Japanese despite commanding 'superior forces' and, despite explicit orders to the contrary, Potts’s failure to launch an offensive to retake Kokoda. Blamey replaced Potts with Brigadier Ivan N. Dougherty, who commanded the 21^st Brigade for the rest of the war. Blamey later addressed the survivors of the 21^st Brigade in a manner so egregiously insensitive that there was almost a mutiny.

The Japanese were meanwhile completing their withdrawal to formidable defences around their Buna and Gona beach-heads, reinforced by fresh Japanese units from Rabaul. The joint Australian and US 'Lilliput' operation was then launched as the preparation for the crushing of the Japanese beach-heads in the Battle of Buna and Gona.

The Kokoda Trail and subsequent New Guinea campaigns were the first time that Australia’s security had been threatened directly. Given that at the time Papua and North-East New Guinea constituted an Australian protectorate, the Kokoda Trail campaign was also the first occasion in which Australians had fought to repel an invader of Australian soil. It was also the first time that Australia had fought without the material presence or support of the UK. During the Kokoda Trail campaign, in which they had committed an initial 2,000 men, the Australians suffered the loss of 625 men killed, 1,055 wounded, and more than 4,000 prostrated by disease. The Japanese had committed an initial 10,000 men, later supplemented by another 3,500, and suffered the loss of 6,600 men killed as well as many hundreds more incapacitated by disease and malnutrition.