'Sob' was the British nickname ('sea or bust') rather than formal designation of the advance of Lieutenant General Sir William Slim’s British 14th Army in Burma from the area of Pakokku, Meiktila and Mandalay to the south down the lines of the Irrawaddy and Sittang rivers toward Rangoon (9 April/6 May 1945).
While the Japanese forces of Lieutenant General Shozo Sakurai’s 28th Army of General Heitaro Kimura’s Burma Area Army were distracted by the assault on Meiktila by Lieutenant General Sir Frank Messervy’s Indian IV Corps in February and March, Lieutenant General Sir Montagu Stopford’s Indian XXXIII Corps had renewed its attack on Mandalay, which lay in the area of responsibility of Lieutenant General Shihachi Katamura’s 15th Army. Mandalay fell to Major General T. W. Rees’s Indian 19th Division on 20 March, though the Japanese managed to hold Fort Dufferin, the former citadel, for another week.
The Japanese decision to seek to hold Mandalay, when the forces of the Indian IV Corps were already well to the south-west and thus across the 15th Army's lines of communication, was senseless, especially after the Indian XXXIII Corps' other divisions (Major General D. D. Gracey’s Indian 20th Division and Major General C. G. G. Nicholson’s British 2nd Division) had advanced simultaneously from their bridgeheads across the Irrawaddy river to advance on Thedaw and Ava respectively, thereby threatening the 15th Army's lines of communication between Mandalay and Meiktila. The inevitable consequence was that the 15th Army was reduced to little more than small detachments of stragglers seeking to make their way to the south, or otherwise to the east into the Shan States, where Lieutenant General Masaki Honda’s 33rd Army, whose headquarters had already belatedly been summoned to co-ordinate the fruitless Japanese effort to retake Meiktila, was currently struggling as its remnants sought to move to the south ahead of the US-led Chinese advances in northern Burma.
The British capture of Mandalay, and of Maymyo to its east, completely severed the Japanese lines of communications to the front in northern Burma, and thereby finally secured the Allied road link between India and China, albeit considerably too late for this to have any real effect the course of the war in China. The fall of Mandalay also precipitated the change of sides by the Burma National Army, and open rebellion against the Japanese by other underground movements belonging to the Anti-Fascist Organisation.
The Allied forces had now successfully completed their 'Extended Capital' advance into central Burma, but it remained vital for them to reach and take Rangoon and its strategically important port facilities before the advent of the torrential rain of the south-west monsoon early in May ended the spring campaigning season of 1945. The rain would literally dissolve the temporarily upgraded overland routes from India, and also severely limit flight operations and so reduce the quantity of supplies which could be delivered by air. Furthermore, Admiral the Lord Louis Mountbatten’s South-East Asia Command had been notified that many of the US transport aircraft allocated to the theatre would be withdrawn in June at the latest. Thus possession of Rangoon and its port facilities was essential to support the requirements of the British-led armed forces and also to provide the food needed by the civilian population in the areas which had been liberated.
At this time two British formations, Nicholson’s 2nd Division and Major General F. W. Festing’s 36th Division, were both under strength and could not readily be reinforced, and were therefore withdrawn to India to reduce the demand for supplies. The Indian XXXIII Corps, comprising Major General G. C. Evans’s Indian 7th Division and Gracey’s Indian 20th Division, now began the 14th Army’s secondary advance toward Rangoon down the Irrawaddy river valley in the face of patchy but determined resistance by the formations of the 28th Army. At the same time the Indian IV Corps, comprising Major General E. C. R. Mansergh’s Indian 5th Division, Major General D. T. Cowan’s Indian 17th Division and Rees’s Indian 19th Division, delivered the main attack in parallel down the Sittang river valley.
The Indian 17th Division and Brigadier C. E. Pert’s Indian 255th Armoured Brigade began the IV Corps' advance on 6 April by striking from all sides at the delaying position held by the remnants of the 33rd Army at Pyawbwe, while the 'Claudcol' flanking column of tanks and mechanised infantry cut the main road behind them and attacked their rear. 'Claudcol' was at first checked by the remnants of Lieutenant General Saburu Takehara’s 49th Division defending a village, but soon bypassed this block to defeat the remnants of Lieutenant General Kaoru Takeda’s 53rd Division and destroy the 14th Tank Regiment’s last tanks. As 'Claudcol' wheeled to the north against the town of Pyawbwe itself, it attacked the location of Honda’s headquarters but, unaware of the fact, broke off to take the town.
From this point, the advance down the main road to Rangoon met little in the way of organised opposition. The town and bridge at Pyinmana were seized on 19 April before the Japanese could organise their defence. Pyinmana was the current location of Honda’s headquarters, and from the information supplied by agents the Indian IV Corps knew of this and therefore attacked the headquarters. Honda and his staff managed to escape during night on foot, but were now left with little by which to control what was left of the 33rd Army.
By this time part of the 15th Army had been reorganised in the Shan States, where it was strengthened by the arrival of Lieutenant General Yuzo Matsuyama�s 56th Division transferred from the northern front. This revived Japanese strength was instructed to move to Toungoo and block the road to Rangoon, but a general uprising by Karen forces, organised and equipped by Force 136, delayed the Japanese long enough for the Indian 5th Division to reach the town first on 23 April. The Japanese briefly recaptured Toungoo once the Indian 5th Division had moved farther to the south, but the Indian 19th Division, which was following in the wake of the Indian IV Corps' leading elements, once more recaptured Toungoo and began to drive the Japanese back toward Mawchi to the east.
At this point the Indian 17th Division took over the lead of the advance, and met a Japanese blocking force to the north of Pegu, just 40 miles (65 km) north of Rangoon, on 25 April. Line of communication troops, naval personnel and even Japanese civilians in Rangoon had meanwhile been formed into the extemporised 105th Independent Mixed Brigade. This used anti-tank mines improvised from aircraft bombs, anti-aircraft guns and suicide attacks with pole charges to delay the Indian division, and with such weapons managed to hold Pegu until 30 April, allowing the Japanese evacuation of Rangoon to the east and the lower reaches of the Sittang river, after which the brigade escaped into the hills to the west of Pegu.
The monsoon broke on 2 May as the Indian 17th Division resumed its advance on Rangoon, and the resulting floods somewhat slowed the division.
On the previous day Rangoon, now empty of the Japanese, had been occupied by forces delivered by sea in 'Dracula' (ii), and elements of Major General C. E. N. Lomax’s Indian 26th Division started to advance to the north-east in the direction of Pegu. Units of the Indian 17th and 26th Divisions met at Hlegu on 6 May.
After the seizure of Rangoon, a new British 12th Army was created under Stopford, on the basis of the Indian XXXIII Corps' headquarters, to assume control of the formations, including the Indian IV Corps, scheduled to remain in Burma as the rest of the British 14th Army was pulled back to India in preparation for the planned 'Zipper' descent on Malaya.
What was left of the Burma Area Army now held the Tenasserim province of south-eastern Burma. The 28th Army, which had withdrawn from Arakan and unsuccessfully resisted the Indian XXXIII Corps in the Irrawaddy river valley, and the 105th Independent Mixed Brigade were cut off in the Pegu Yomas, a range of low but thickly jungled hills between the Irrawaddy and Sittang rivers, from which they were to break out, cross the Sittang river and rejoin the Burma Area Army. In order to provide cover for this break-out, Kimura ordered the 33rd Army to make a diversionary offensive across the Sittang river, although the entire army now had the strength of barely one regiment. On 3 July this force attacked the British positions in the so-called 'Sittang bend'. After a battle for country which was almost entirely under chest-high water, on 10 July both Brigadier W. A. Crowther’s Indian 89th Brigade and the further decimated Japanese pulled back.
Honda had attacked too soon, for the remnants of Sakurai’s 28th Army was not ready to start the break-out from the Pegu Yomas until 17 July, when there followed total disaster for the Japanese. The British had captured the Japanese plans from an officer killed making a final reconnaissance and had sited ambushes and pre-registered artillery bombardments on the routes the Japanese were to use, inflicting major losses on Japanese troops who were starving, afflicted by disease and lacking all but some personal weapons. Hundreds more drowned while attempting to cross the swollen Sittang river on improvised bamboo floats and rafts, and Burmese guerrillas and bandits killed many stragglers who managed to get across the river. The break-out cost the Japanese nearly 10,000 men, which was about half of the 28th Army's surviving strength, and some of the 105th Independent Mixed Brigade's units were almost totally destroyed. The British and Indian casualties were minimal.
Now commanded by Lieutenant General Sir Miles Dempsey, the 14th Army and Lieutenant General Sir Philip Christison’s Indian XV Corps, which had been operating in the Arakan western coastal region, had returned to India to plan 'Zipper', which was to be the next stage of the campaign to retake South-East Asia, and a new formation, Lieutenant General O. L. Roberts’s Indian XXXIV Corps, was created and allocated to the 14th Army for these operations. However, the need for 'Zipper' was removed by the Japanese surrender in August 1945, but the core of the operation was then undertaken as the swiftest way to deliver occupation formations into Malaya.