The Burma campaign was a series of battles fought in the British colony of Burma. It was part of the South-East Asian theatre of World War II and primarily involved forces of the Allies; the British Empire and the Republic of China, with support from the United States. They faced against the invading forces of Imperial Japan, who were supported by the Thai Phayap Army, as well as two collaborationist independence movements and armies, the first being the Burma Independence Army, which spearheaded the initial attacks against the country. Puppet states were established in the conquered areas and territories were annexed, while the international Allied force in British India launched several failed offensives. During the later 1944 offensive into India and subsequent Allied recapture of Burma the Indian National Army, led by revolutionary Subhas C. Bose and his "Free India", were also fighting together with Japan. British Empire forces peaked at around 1,000,000 land and air forces, and were drawn primarily from British India, with British Army forces (equivalent to eight regular infantry divisions and six tank regiments), 100,000 East and West African colonial troops, and smaller numbers of land and air forces from several other Dominions and Colonies.The campaign had a number of notable features. The geographical characteristics of the region meant that weather, disease and terrain had a major effect on operations. The lack of transport infrastructure placed an emphasis on military engineering and air transport to move and supply troops, and evacuate wounded. The campaign was also politically complex, with the British, the United States and the Chinese all having different strategic priorities.

It was also the only land campaign by the Western Allies in the Pacific Theatre which proceeded continuously from the start of hostilities to the end of the war. This was due to its geographical location. By extending from South East Asia to India, its area included some lands which the British lost at the outset of the war, but also included areas of India wherein the Japanese advance was eventually stopped.

The climate of the region is dominated by the seasonal monsoon rains, which allowed effective campaigning for only just over half of each year. This, together with other factors such as famine and disorder in British India and the priority given by the Allies to the defeat of Nazi Germany, prolonged the campaign and divided it into four phases: the Japanese invasion, which led to the expulsion of British, Indian and Chinese forces in 1942; failed attempts by the Allies to mount offensives into Burma, from late 1942 to early 1944; the 1944 Japanese invasion of India, which ultimately failed following the battles of Imphal and Kohima; and finally the successful Allied offensive which liberated Burma from late-1944 to mid-1945.

The campaign was also strongly affected from the political atmosphere which erupted in the South-East Asian regions occupied by Japan, who pursued the Pan-Asianist policy of a "Greater East Asia Co-Prosperity Sphere". These led to a Japanese-sponsored revolution during the initial invasion and the establishment of the State of Burma, in which the Provisional Government of Free India, with its Indian National Army, was headquartered. The dominating attitude of the Japanese militarist who commanded the army stationed in the country, which ultimately doomed the co-prosperity sphere as a whole, led to local hopes for real Independence fade and the war-time established Burma National Army revolted in 1945. On the Allied side political relations were mixed for much of the war. The China Burma India Theater American-trained Chinese X Force led to cooperation between the two countries, but the clashing strategies proposed by "Vinegar Joe" Stilwell and Chinese Generalissimo Chiang Kai-shek would lead to Stilwell's eventual removal from his position as American Commander of the theater. On the other hand, ChinaIndia relations were positive from the cooperative Burma Road, built to reach the Chinese Y Force and the Chinese war effort inside of China, as well as from the heroic missions over the extremely dangerous air route over the Himalayas, nicknamed "The Hump". The campaign would have a great impact on the independence struggle of Burma and India in the post-war years.

Phayap Army (Thai: กองทัพพายัพ RTGS: Thap Phayap or Payap, northwest) was the Thai force that invaded the Siamese Shan States (present is Shan States) of Burma on 10 May 1942 during the Burma Campaign of World War II.