Towards the end of World War II, what army became the largest volunteer army in world history, with over 2.5 million?

In World War II Indian soldiers fought with the Allies. In 1939, British officials had no plan for expansion and training of Indian forces, which comprised about 130,000 men (in addition there were 44,000 men in British units in India in 1939). Their mission was internal security and defence against a possible Soviet threat through Afghanistan. As the war progressed, the size and role of the Indian Army expanded dramatically, and troops were sent to battlefronts as soon as possible. The most serious problem was lack of equipment.

Indian units served in Burma, where in 1944 - 45, five Indian divisions were engaged along with one British and three African divisions. Even larger numbers operated in the Middle East. Some 87,000 Indian soldiers died in the war. By the end of the war it had become the largest volunteer army in history, rising to over 2.5 million men in August 1945.