Before the Second World War broke out, Burma was part of the British Empire, having been progressively occupied and annexed following three Anglo-Burmese wars in the 19th century. Initially governed as part of British India, Burma was formed into a separate colony under the Government of India Act 1935. Under British rule, there had been substantial economic development but the majority Bamar community was becoming increasingly restive. Among their concerns were the importation of Indian workers to provide a labour force for many of the new industries, and the erosion of traditional society in the countryside as land was used for plantations of export crops or became mortgaged to Indian moneylenders. Pressure for independence was growing. When Burma came under attack, the Bamar were unwilling to contribute to the defence of the British establishment, and many readily joined movements which aided the Japanese.
British plans for the defence of British Far Eastern possessions involved the construction of airfields linking Singapore and Malaya with India. These plans had not taken into account the fact that Britain was also at war with Germany, and when Japan entered the war, the forces needed to defend these possessions were not available. Burma had been regarded as a military "backwater", unlikely to be subjected to Japanese threat.
Lieutenant General Thomas Hutton, the commander of Burma Army with its headquarters in Rangoon, had only the 17th Indian Infantry Division and 1st Burma Division to defend the country, although help was expected from the Chinese Nationalist government under Chiang Kai-shek. During the war, the British Indian Army expanded more than twelve-fold from its peacetime strength of 200,000 but in late 1941 this expansion meant that most units were undertrained and ill-equipped. In most cases, such training and equipment as the Indian units in Burma received was for operations in the Western Desert campaign or the North West Frontier of India, rather than jungles. The battalions of the Burma Rifles which formed most of the 1st Burma Division were originally raised as internal security troops only, from among minority communities in Burma such as the Karens. They also had been rapidly expanded, with an influx of Bamar soldiers, and were short of equipment and consisted mainly of new recruits.
Japan entered the war primarily to obtain raw materials, especially oil, from European (particularly Dutch) possessions in South East Asia which were weakly defended because of the war in Europe. Their plans involved an attack on Burma partly because of Burma's own natural resources (which included some oil from fields around Yenangyaung, but also minerals such as cobalt and large surpluses of rice), but also to protect the flank of their main attack against Malaya and Singapore and provide a buffer zone to protect the territories they intended to occupy.
An additional factor was the Burma Road completed in 1938, which linked Lashio, at the end of a railway from the port of Rangoon, with the Chinese province of Yunnan. This newly completed link was being used to move aid and munitions to the Chinese Nationalist forces of Chiang Kai-Shek which had been fighting the Japanese for several years. The Japanese naturally wished to cut this link.
The Japanese Fifteenth Army, commanded by Lieutenant General Shojiro Iida, was assigned the mission of occupying northern Thailand, which had signed a treaty of friendship with Japan on 21 December 1941, and attacking the southern Burmese province of Tenasserim across the Tenasserim Hills. The army consisted initially of the highly regarded 33rd Division and the 55th Division, although both divisions were weakened for several weeks by detachments to other operations.
As the threat of war grew, the Japanese sought links with potential allies in Burma. In late 1940 Aung San, a Burmese student activist, made contact with Japanese officers on Amoy and was flown to Japan for talks. He and several other volunteers (the "Thirty Comrades") were later given intensive military training on Hainan Island.
The Burma Independence Army was officially founded in Bangkok on 28 December 1941. It consisted initially of 227 Burmese and 74 Japanese personnel but were rapidly reinforced by large numbers of volunteers and recruits once they crossed into Burma as part of the main Japanese invasion.
The first Japanese attack in mid-January 1942 against Victoria Point, almost the most southerly point of Burma, was expected and was not contested. The second attack was a small probing raid directed at a police station in southern Tenasserim, which was repulsed. The Japanese 143 Infantry Regiment (from 55th Division) then launched overland attacks on the airfields at Tavoy and Mergui in Tenasserim. The airfields were difficult to defend and reinforce, but Burma Army HQ had been ordered to hold these outposts because of their importance to the defence of Malaya. The Japanese forced their way over the steep jungle-covered Tenasserim Range, and attacked Tavoy on 18 January. The defenders, the 3rd and 6th battalions of the Burma Rifles, were overwhelmed and forced to evacuate the town in disorder. Mergui was evacuated before it was attacked.
Rangoon was initially defended relatively successfully against Japanese air raids, by small RAF detachments reinforced by a squadron of the American Volunteer Group, better known as the "Flying Tigers". The majority of the airfields were between Rangoon and the Axis advance and as the Japanese gained use of the airfields in Tenasserim, the amount of warning the Rangoon airfields could get of attack decreased, and they became more and more untenable.
On 22 January 1942, the main body of the Japanese 55th Division began the main attack westward from Rahaeng in Thailand across the Kawkareik Pass. The 16th Indian Infantry Brigade of the 17th Indian Division guarding this approach retreated hastily westward. The Japanese division advanced to Moulmein at the mouth of the Salween River which was garrisoned by the 2nd Burma Infantry Brigade. The position was almost impossible to defend, and had the River Salween, almost 1.5 miles (2.4 km) wide, behind it. The 2nd Burma Brigade was squeezed into a progressively tighter perimeter, and eventually retreated over the river by ferry on 31 January after abandoning a large amount of supplies and equipment. Part of the force was left behind in Moulmein and had to swim the river.
The 17th Indian Division fell back northward. They attempted to hold the Bilin River and other fallback lines as they did so, but had too few troops to avoid being continually outflanked. The division eventually retreated toward the bridge over the Sittang River in general disorder. The retreat was delayed by incidents such as a vehicle breaking through the bridge deck, air attacks (including, allegedly, accidental attacks by the RAF and AVG), and Japanese harassment. The delays allowed Japanese parties to infiltrate to the bridge itself, and the poorly organised defence of the bridge was in danger of collapsing. Fearing that the bridge would fall intact to the Japanese who would use it to advance on Rangoon, the divisional commander, Major-General "Jackie" Smyth, VC, ordered it to be blown up on the morning of 23 February 1942, with most of the division stranded on the enemy-held side.
Many of the men of the 17th Division who were trapped on the Japanese-held side of the river made their way across to the west bank by swimming or on improvised rafts, but had to abandon almost all their equipment, including most of their small arms. This later led some to question the decision to blow the bridge, arguing that the river itself did not offer much of an obstacle to the Japanese, and that more harm than good was achieved, as it resulted in the stranding of two brigades and delayed the Japanese capture of Rangoon by ten days at most.
Though the Sittang River was in theory a strong defensive position, the disaster at the bridge left the Allied forces too weak to hold it. General Archibald Wavell, the commander-in-chief of the ABDA Command, nevertheless ordered Rangoon to be held. He was expecting substantial reinforcements from the Middle East, including an Australian infantry division. On 28 February, he formally relieved Hutton (although Hutton had officially already been superseded in command by General Harold Alexander), and on the following day he sacked Smyth, who was in any case very ill.
Although the Australian government refused to allow its troops to be committed to Burma, some British and Indian reinforcements, including the British 7th Armoured Brigade (equipped with new M3 Stuart tanks) and the 63rd Indian Infantry Brigade, landed in Rangoon. Alexander ordered counter-attacks against the Japanese at Pegu, 40 miles (64 km) northeast of Rangoon, but soon realised that there was no hope of defending Rangoon. On 7 March, the Burma Army evacuated Rangoon after implementing a scorched earth plan to deny the Japanese the use of its facilities. The port was destroyed and the oil terminal was blown up. As the Allies departed, the city was on fire.
The remnants of the Burma Army faced encirclement as they retreated north from the city, but broke through the Taukkyan Roadblock as the result of an error on the part of the local Japanese commander. Colonel Takanobu Sakuma, commanding the Japanese 214th Infantry Regiment, had been ordered to block the main road north from Rangoon to Prome while the main body of the 33rd Division circled round the city to attack from the west. The retreating British and Indian troops were thrown back when they attempted to break through Sakuma's road block. Alexander ordered another attack but found the Japanese had gone. Not realising that the British were evacuating Rangoon, Sakuma had withdrawn the road block, as ordered, once the 33rd Division reached its intended positions. Had he not done so, the Japanese might have captured General Alexander and much of the rest of Burma Army.
After the fall of Rangoon, the Allies tried to make a stand in central Burma. It was hoped that the Chinese Expeditionary Force in Burma, commanded by Luo Zhuoying and consisting of the Fifth Army (commanded by Du Yuming) and the Sixth and Sixty-sixth Armies, could hold a front south of Mandalay. The Chinese armies each had approximately the strength of a British division but comparatively little equipment. Meanwhile, the newly created Burma Corps which had been formed to relieve Burma Army headquarters of the day-to-day responsibility for operations and consisted of 1st Burma Division, 17th Indian Division and 7th Armoured Brigade, defended the Irrawaddy River valley. Supplies were not immediately a problem, as much war material (including material originally meant for shipment to China) had been evacuated from Rangoon, rice was plentiful and the oilfields in central Burma were still intact, but no proper land routes from India existed and only the recapture of Rangoon would allow the Allies to hold Burma indefinitely.
The Allies hoped that the Japanese advance would slow down; instead, it gained speed. The Japanese reinforced their two divisions in Burma with one transferred from Malaya and another transferred from the Dutch East Indies after the fall of Singapore and Java. They also brought in large numbers of captured British trucks and other vehicles, which allowed them to move supplies rapidly using southern Burma's road network, and also use Motorised infantry columns, particularly against the Chinese forces. The Royal Air Force wing operating from Magwe was crippled by the withdrawal of the radar and radio-intercept units to India and the Japanese soon gained supremacy in the air. Unopposed Japanese bomber fleets attacked almost every major town and city in the Allied-held part of Burma, causing widespread destruction and disorder. The Allies were also harassed by the rapidly expanding Burma Independence Army, and were hampered by large numbers of refugees (mostly Indian civilians) and the progressive breakdown of the civil government in the areas they held. Many Bamar soldiers of the Burma Rifles also were deserting.
The commander of Burma Corps, Lieutenant General William Slim, tried to mount a counter-offensive on the western part of the front, but his troops were repeatedly outflanked and forced to fight their way out of encirclement. The Corps was gradually pushed northward towards Mandalay. The 1st Burma Division was cut off and trapped in the blazing oilfields at Yenangyaung, which the Allies themselves demolished to deny the facilities to the Japanese. Although the division was rescued by Chinese infantry and British tanks in the Battle of Yenangyaung, it lost almost all its equipment and its cohesion.
On the eastern part of the front, in the Battle of Yunnan-Burma Road, the Chinese 200th Division held up the Japanese for a time around Toungoo, but after its fall the road was open for motorised troops of the Japanese 56th Division to shatter the Chinese Sixth Army to the east in the Karenni States and advance northward through the Shan States to capture Lashio, outflanking the Allied defensive lines and cutting off the Chinese armies from Yunnan. With the effective collapse of the entire defensive line, there was little choice left other than an overland retreat to India or to Yunnan.
The retreat was conducted in horrible circumstances. Starving refugees, disorganised stragglers, and the sick and wounded clogged the primitive roads and tracks leading to India. The Burma Corps retreated to Manipur in India. At one stage, Alexander proposed that the 7th Armoured Brigade and one infantry brigade accompany the Chinese armies into Yunnan, but he was persuaded that the armoured brigade would quickly become ineffective once it was cut off from India.
The Japanese tried to cut off Burma Corps by sending troops by boat up the Chindwin River to seize the riverside port of Monywa on the night of 1/2 May. As Burma Corps tried to cross to the west bank of the Chindwin by ramshackle ferries to Kalewa, on 10 May the Japanese advancing from Monywa attempted to surround them in a "basin" encircled by cliffs at Shwegyin on the east bank. Although counter-attacks allowed the troops to escape, most of the Burma Corps' remaining equipment had to be destroyed or abandoned. The Corps reached Imphal in Manipur just before the monsoon broke in May 1942. There, they found themselves living out in the open under the torrential monsoon rains in extremely unhealthy circumstances. The army and civil authorities in India were very slow to respond to the needs of the troops and civilian refugees. The ad hoc Burma Corps HQ was disbanded and IV Corps HQ, which had recently arrived in India, took over the front.
The British Civil Government of Burma fell back to Myitkyina in Northern Burma, accompanied by many British, Anglo-Indian and Indian civilians. The Governor (Reginald Dorman-Smith) and the most influential civilians were flown out from Myitkyina Airfield, together with some of the sick and injured. The majority of the refugees were forced to make their way from Myitkyina to India via the unhealthy Hukawng Valley and the precipitous forested Patkai Range. Many died on the way, and when they reached India, there were several instances in which civil authorities allowed white and Eurasian civilians to continue while preventing Indians from proceeding, effectively condemning many to death. By contrast, many private individuals such as tea planters did their best to provide aid.
The Japanese advance cut off many of the Chinese troops from China. Many of them also retreated via the Hukawng Valley route and subsisted largely by looting, further increasing the misery of the refugees. The Chinese 38th Division however, commanded by Sun Li-jen, fought its way westward across the Chindwin, arriving in India substantially intact although with heavy casualties. The Chinese soldiers who had retreated into India were put under the command of the American General Joseph Stilwell, who had also made his way to India on foot, and were concentrated in camps at Ramgarh in Bihar. After recuperating they were re-equipped and retrained by American instructors. The remaining Chinese troops tried to return to Yunnan through remote mountainous forests and many died on the way.
The Japanese 18th and 56th Divisions pursued the Chinese into Yunnan, but were ordered to halt on the Salween River on 26 April. The Japanese 33rd Division likewise halted on the Chindwin at the end of May, ending the campaign until the end of the monsoon rains. In the coastal Arakan Province, some of the Burma Independence Army reached Akyab Island before the Japanese troops. However, they also instigated riots between the Buddhist and Moslem populations of the province. The Japanese advance in Arakan ended just south of the Indian frontier, prompting the British military and civil authorities in and around Chittagong to implement a premature "scorched earth" policy which contributed to the Bengal Famine of 1943.
In accordance with the Thai military alliance with Japan that was signed on 21 December 1941, When Thailand signed the Tripartite Pact on 15 February 1942, the Thais and Japanese also agreed that the Eastern part of the Karenni States and Shan State were to be under Thai control. The rest of Burma was to be under Japanese control.
The leading elements of the Thai Phayap Army (commanded by Jarun Rattanakun Seriroengrit) crossed the border into the Shan States on 10 May 1942. Three Thai infantry division and one cavalry division, spearheaded by armoured reconnaissance groups and supported by the air force, engaged the retreating Chinese 93rd Division. Kengtung, the main objective, was captured on 27 May.
On 12 July, Major General Phin Choonhavan, the Thai military governor of Shan State, ordered the 3rd Division of the Phayap Army to occupy Karenni States and expel the Chinese 55th Division from Loikaw. The Chinese troops could not retreat because the routes to Yunnan were controlled by the Thais and Japanese. The Thais captured many Chinese soldiers.