The Burma Campaign in World War II lasted nearly as long as the involvement of the United States and Britain in the war in the Pacific, from January 1942 to after V-E Day. The adjective "forgotten" is often included in references to the campaign itself or the armies that fought in it; Burma was considered a secondary theatre at best, not as important as Europe or the American island hopping campaign in the Pacific. But it saw some of the hardest fighting of the war, and brought Japanese forces all the way to India, from where they might easily have marched on Delhi and dissolved the British Raj if the campaign had gone differently.
Japan launched an attack into Burma, in January 1942, shortly after Pearl Harbor. At this point, Japanese forces had been engaged in fighting across Asia for nearly ten years, and most of their energies were absorbed with attempting to conquer China; while they controlled the Chinese coast, Chinese Communists under Mao Zedong and nationalists under Chiang Kai-Shek continued to resist them from interior areas. Chiang, obviously favoured most by the West because he wasn't a Communist, had his base in Chongqing in south-west China, from where he received Western supplies and aid by overland routes that ran from India and Burma, both of which were British colonies at the time. It was these routes that the Japanese hoped to cut off.
The Japanese seized Rangoon, which today is the capital of independent Burma, easily and drove inexperienced British forces out of the country. The Burmese population were known to be hostile to their colonizers, and a substantial number of native troops helped the Japanese - as elsewhere in Asia, the Japanese used opposition to colonialism by Western countries to mask their own imperial designs. The white man fled to safer ground in India, but the Japanese quickly achieved their objective of cutting off overland transport to Chiang's forces in China. American planes soon had to start flying over steep mountains on a journey they called "the Hump" to transport supplies to the Chinese nationalists.
The Allies, and particularly the U.S., were obsessed with reopening the overland route to China. They believed that the Chinese Nationalists were tying down substantial Japanese forces by fighting them in China, and that these forces could be diverted to counter the American island-hopping campaign in the Pacific if they were not kept there. American strategic planners also dreamed of the day that Japanese forces would be routed in China and they could use airbases there to bomb the Japanese mainland, hence contributing to final victory. The campaign in China was hence valued mainly for the support it provided to the war against the Japanese homeland itself; and the campaign in Burma was in turn valued mainly for the support it provided to the effort in China. It was far down the priority list indeed.
This did not make it any easier for the men who fought and died there. After retreating from Burma, the British and Americans established a base in northern India around two towns called Imphal and Kohima. From here they intended to counterattack against the Japanese, and they made one early foray in Burma which came to be known as the Battle of the Admin Box. This uninspiring name had nothing to do with paperwork, but was rather a reference to a defensive box that the Allied forces established in which they could be completely supplied by airdrops of material. The concept worked, whereas Japanese forces had little air support and were told by their commanders to capture supplies from the enemy. Save for the occasional stray airdrop - which the Japanese dubbed "Churchill rations", although their reaction to one of Western civilization's horrors, corned beef, can only be imagined - Japanese forces soon found themselves starving and running low on ammunition.
As well as air supply, the Burma Campaign also saw the development and deployment of a quasi-guerrilla force which the Allies called the Chindits. Designed to operate behind enemy lines in large numbers for prolonged periods, and again be supplied from the air, Chindit units were dispatched to interrupt Japanese supply lines and rear-echelon units. One unit, named Merrill's Marauders after their commander, marched 1,000 miles behind Japanese lines and launched withering attacks; while their contribution to the campaign is disputed, they served as a prototype for the eventual creation of the United States Army Rangers.
Having run afoul of the Allies at the Battle of the Admin Box, the Japanese nevertheless continued with plans for a major offensive into northern India designed to further disrupt Allied plans for counter-attacks, to cut off "the Hump" route which was still managing to keep the Chinese nationalists relatively well-supplied, and perhaps to march all the way to Delhi. The Japanese surprised the Allies by invading India in strength, placing Kohima and Imphal under prolonged siege. Only by a historically unprecedented use of airplanes to redeploy entire divisions of forces and to bring in hundreds of tonnes of supplies a day to the besieged garrisons - including some 40 million cigarettes over the course of the battles - did the Allies manage to hold out.
The Japanese were bled white in some of the worst battles of the war around Kohima and Imphal. The fighting was as long and bitter as it was primarily because of the Japanese aversion to surrendering or admitting defeat, which would lead completely doomed units to fight to the last man, inflicting horrific casualties on the Allies. As before, Japanese commanders had expected their units to capture supplies from Imphal and Kohima and had not prepared for a prolonged stalemate; their fighters were soon emaciated and wracked with tropical diseases. The monsoon brought rain and mud, creating scenes that veterans compared to those of the Battle of the Somme in World War I.
Eventually the Japanese broke, and were pursued by the Allies back into Burma and ejected altogether. Although the campaign represented the biggest loss for the Japanese anywhere so far, it was largely irrelevant to the outcome of the war as a whole; the land route to China was not reopened until the war was at such a late stage that it hardly mattered anymore. The Allies retook Rangoon almost exactly as the German surrender in Europe was being accepted, and one can imagine the priorities of the newspapers when both bits of news broke; meanwhile, both Burma and India were to be independent in several years despite the huge cost in lives expended in defending them for the British Empire. The cost in lives of this sideshow was nearly 300,000 - a measure of the insanity of a world in the midst of total war.