During the Second World War (1939–1945), India was a part of the British Empire. British India officially declared war on Nazi Germany in September 1939. India, as a part of the Allied Nations, sent over two and a half million soldiers to fight under British command against the Axis powers. India was also used as the base for American operations in support of China in the China Burma India Theater.

Indians fought throughout the world, including in the European theatre against Germany, North African Campaign against fascist Italy, and in the southeast Asian theatre; while also defending the Indian subcontinent against the Japanese forces, including British Burma and the Crown colony of Ceylon. Indian troops were also redeployed in former colonies such as Singapore and Hong Kong, with the Japanese surrender in August 1945, after the end of World War II. Over 87,000 Indian troops, and 3 million civilians died in World War II.Commonwealth War Graves Commission Annual Report 2013-2014 , page 44. Figures include identified burials and those commemorated by name on memorials. Field Marshal Sir Claude Auchinleck, former Commander-in-Chief, India, stated that Britain "couldn't have come through both wars [World War I and II] if they hadn't had the Indian Army."

There was pushback throughout India to expending lives supporting the colonial British Empire in Africa and Europe amidst movements for Indian independence. Particularly, Subhas Chandra Bose sought alliance with the Soviet Union and then ultimately with Nazi Germany as a tool for subverting the British empire. Many factions of the Indian Independence Movement did support Nazi Germany during the war, most notably the so-called Indian Legion which Bose was instrumental in creating and which was incorporated for some time as a division of the Waffen-SS.

The Marquess of Linlithgow, Viceroy of India, declared war on Germany on India's behalf in September 1939, an action for which he was criticised by elements of the Indian National Congress for not consulting the Indian people. Political parties such as the Muslim League and the Hindu Mahasabha supported the British war effort while the largest and most influential political party existing in India at the time, the Indian National Congress, demanded independence before it would help Britain. London refused, and when Congress announced a "Quit India" campaign in August 1942, tens of thousands of its leaders were imprisoned by the British for the duration. Meanwhile, under the leadership of Indian leader Subhas Chandra Bose, Japan set up an army of Indian POWs known as the Indian National Army which fought against the British. In 1943, a major famine in Bengal led to between 0.8 and 3.8 million deaths due to starvation, and a highly controversial issue remains regarding Winston Churchill's decision not to provide emergency food relief.

Indian participation in the Allied campaign remained strong. The financial, industrial and military assistance of India formed a crucial component of the British campaign against Nazi Germany and Imperial Japan. India's strategic location at the tip of the Indian Ocean, its large production of armaments, and its huge armed forces played a decisive role in halting the progress of Imperial Japan in the South-East Asian theatre. The Indian Army during World War II was one of the largest Allied forces contingents which took part in the North and East African Campaign, Western Desert Campaign. At the height of the second World War, more than 2.5 million Indian troops were fighting Axis forces around the globe. After the end of the war, India emerged as the world's fourth largest industrial power and its increased political, economic and military influence paved the way for its independence from the United Kingdom in 1947. The Indian servicemen who served in the British Indian Army, the Royal Indian Navy, and the Indian Air Force during World War II and still had service period remaining at the time of India's Independence would go on to become serving members of the future armies, navies, and air forces of post-Partition India and Pakistan.

Quit India movement

The Indian National Congress, led by Mohandas Karamchand Gandhi, Vallabhbhai Patel and Maulana Azad, denounced Nazi Germany but would not fight it or anyone else until India was independent. Congress launched the Quit India Movement in August 1942, refusing to co-operate in any way with the government until independence was granted. The government, not ready for this, immediately arrested over 60,000 national and local Congress leaders, and then moved to suppress the violent reaction of Congress supporters. Key leaders were kept in prison until June 1945, although Gandhi was released in May 1944 because of his health. Congress, with its leaders incommunicado, played little role on the home front. Unlike the predominately Hindu Congress, the Muslim League rejected the Quit India movement and worked closely with the Raj authorities.

Supporters of the British Raj argued that decolonisation was impossible in the middle of a great war. So, in 1939, the British Viceroy, Lord Linlithgow declared India's entry into the War without consulting prominent Indian Congress leaders who were just elected in previous elections.

Subhas Chandra Bose (also called Netaji) had been a top Congress leader. He broke up with the Congress and tried to form a military alliance with Germany or Japan to gain independence. Bose, with the assistance of Germany, formed the Indian Legion from Indian students in Axis occupied Europe and Indian Army prisoners of war. With German reversals in 1942 and 1943, Bose and the Legion's officers were transported by U boat to Japanese territory to continue his plans. Upon arrival, Japan helped him set up the Indian National Army (INA) which fought under Japanese direction, mostly in the Burma Campaign. Bose also headed the Provisional Government of Free India, a government-in-exile based in Singapore. It controlled no Indian territory and was used only to raise troops for Japan.Leonard A. Gordon, Brothers Against the Raj: A Biography of Indian Nationalists Sarat & Subhas Chandra Bose (2000)

British Indian Army

In 1939 the British Indian Army numbered 205,000 men. It took in volunteers and by 1945 was the largest all-volunteer force in history, rising to over 3.35 million men., p. 1 These forces included tank, artillery and airborne forces.

Over World War II, officers and soldiers of the British Indian Army received 6,000 awards for gallantry. These included a total of 31 Victoria Crosses, the highest award in British and Commonwealth Forces for operational gallantry. British, Indian, Nepalese as well as future Pakistani soldiers and officers of the British Indian Army taken together account for these Victoria Crosses, as well for the other thousands of gallantry awards attributed to the British Indian Army. Indians were also awarded the George Cross, the highest award for non-operational gallantry and equivalent to the Victoria Cross. Significantly, in World War II, several Indian officers also received the Distinguished Service Order (DSO). The DSO was a level 2A order-cum-decoration, second only to the Victoria Cross, that could be bestowed only upon commissioned officers for operational gallantry and leadership under conditions of actual combat. The DSO became available to Indian officers largely following the gradual Indianisation of the officer ranks of the British Indian Army 1923 onwards. A rare exception was Hiraji Cursetji, an Indian military surgeon who was awarded the DSO during World War I. Level 3A decorations awarded to Indian officers in World War II included the Distinguished Service Cross (DSC), the Military Cross (MC), and the Distinguished Flying Cross (DFC). Other high military recognitions included appointments as officers or members of the Order of the British Empire (shortened as OBE and MBE respectively), both of which ranked below the DSO but above the level 3A decorations.

South-East Asian theatre

The British Indian Army was the key British Empire fighting presence in the Burma Campaign. The Royal Indian Air force's first assault mission was carried out against Japanese troops stationed in Burma. The British Indian Army was key to breaking the siege of Imphal when the westward advance of Imperial Japan came to a halt.

The formations included the Indian III Corps, IV Corps, the Indian XXXIII Corps and the Fourteenth Army. As part of the new concept of Long Range Penetration (LRP), Gurkha troops of the Indian Army were trained in the present state of Madhya Pradesh under their commander (later Major General) Orde Charles Wingate.

These troops, popularly known as Chindits, played a crucial role in halting the Japanese advance into South Asia.

Capture of Indian territory

By 1942, neighbouring Burma was invaded by Japan, which by then had already captured the Indian territory of Andaman and Nicobar Islands. Japan gave nominal control of the islands to the Provisional Government of Free India on 21 October 1943, and in the following March, the Indian National Army with the help of Japan crossed into India and advanced as far as Kohima in Nagaland. This advance on the mainland of South Asia reached its farthest point on Indian territory, with the Japanese finally retreating from the Battle of Kohima and near simultaneous Battle of Imphal in June 1944.

Recapture of Axis-occupied territory In 1944–45, Japan was under heavy air bombardment at home and suffered massive naval defeats in the Pacific. As its Imphal offensive failed, harsh weather and disease and withdrawal of air cover (due to more pressing needs in the Pacific) also took its toll on the Japanese and remnants of the INA and the Burma National Army. In spring 1945, a resurgent British army recaptured the occupied lands.Edward M. Young and Howard Gerrard, Meiktila 1945: The Battle To Liberate Burma (2004)

Notable Indian recipients of gallantry awards For the highest gallantry in the South East Asian theatre, the Victoria Cross was awarded to Fazal Din, Abdul Hafiz, Karamjeet Judge, Ganju Lama, Bhandari Ram, Sher Shah, Gian Singh, Nand Singh, Parkash Singh, Prakash Singh Chib, Ram Sarup Singh, and Umrao Singh. In the same theatre, the George Cross was awarded to Mateen Ansari, Islam-ud-Din, Abdul Rahman, and Mahmood Khan Durrani (future Pakistan Army officer).

For their gallant leadership and personal bravery in different battles during the Burma Campaign, the DSO was awarded to Kondandera Subayya Thimayya (future COAS, Indian Army), Lionel Protip Sen (future CGS, Indian Army), S.P.P. Thorat (future CGS, Indian Army), Harbans Singh Virk, and Sarbjit Singh Kalha. Siri Kanth Korla, a future division commander of the Indian Army during the 1965 Indo-Pakistani War, was awarded the DSO, the MC, and two Mentions in Dispatches for gallantry in different episodes of this Campaign. The Indian DSO winners in Burma also included Akbar Khan, future leader of Pakistani irregular forces in the Indo-Pakistani War of 1947-48 as well as a future CGS, Pakistani Army.

Gurbakhsh Singh was awarded the DSO and appointed as an OBE for leading a contingent the British Indian Army in Japanese captivity in Singapore, and for preventing defection of his men to the Indian National Army during this internment. Abrar Hussain, a future divisional commander of the Pakistan Army in the 1965 Indo-Pakistani war, was appointed as an MBE for a similar feat with interned personnel of the 2/10 Baluch in Singapore.

K.M. Cariappa, the future second Field Marshal of the Indian Army, was appointed as an OBE for non-combat service in this theatre.

Miyan Hayaud Din, another future CGS, Pakistan Army, was appointed as MBE and awarded the MC for his actions in Burma.