Japan, as well as South East Asia was a major refuge for Indian nationalists living in exile before the start of World War II who formed strong proponents of militant nationalism and also influenced Japanese policy significantly. Although Japanese intentions and policies with regards to India were far from concrete at the start of the war, Japan had sent intelligence missions, notably under Major I Fujiwara, into South Asia even before the start of the World War II to garner support from the Malayan Sultans, overseas Chinese, the Burmese resistance and the Indian movement. These missions were successful establishing contacts with Indian nationalists in exile in Thailand and Malaya, supporting the establishment and organisation of the Indian Independence League.
At the outbreak of World War II in South East Asia, 70,000 Indian troops were stationed in Malaya. After the start of the war, Japan's spectacular Malayan Campaign had brought under her control considerable numbers of Indian Prisoners of War, notably nearly 55,000 after the Fall of Singapore. The conditions of service within the British Indian Army as well as the conditions in Malaya had fed dissension among these troops. From these troops, the First Indian National Army was formed under Mohan Singh and received considerable Japanese aid and support. It was formally proclaimed in September 1942 and declared the subordinate military wing of the Indian Independence League in June that year. The unit was dissolved in December 1942 and Mohan Singh was arrested and exiled to Pulau Ubin after apprehensions of Japanese motives with regards to the INA led to disagreements, distrust and subsequently open hostility between Capt. Mohan Singh and INA leadership on one hand, and the leagues leadership, most notable Rash Behari Bose and the Japanese military command on the other. A large number of the initial volunteers chose to revert to Prisoner of War Status and large number of these were subsequently sent to work in the Death Railway or in New Guinea. From the end of December 1942 to February Rash Behari Bose struggled to hold the INA together.
Although there are slight variations in estimates, the INA is considered to have comprised about 40,000 troops when it was disbanded. The following is an estimate attributed to Lt. Colonel G.D. Anderson of British intelligence:
There were 45,000 Indian troops from Malaya captured and assembled in Singapore when the Japanese captured it. Of these, about 5,000 refused to join the INA. The INA at this time had 40,000 recruits. The Japanese were prepared to arm 16,000. When the "first INA" collapsed, about 4,000 withdrew. The Second INA, commanded by Subhash Chandra Bose, started with 12,000 troops. Further recruitment of ex-Indian army personnel added about 8,000-10,000. About 18,000 Indian civilians enlisted during this time. In 1945, at the end of the INA, it consisted of about 40,000 soldiers.