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To understand India in 1942 you have to start with the map of the Second World War. The war was not something happening only in Europe. Japan was advancing rapidly through South-East Asia. Malaya and Singapore fell. Japan pushed into Burma as well. With that, an attack on India stopped being a hypothesis and became a real possibility.
At that point India was not merely a colony to Britain. Hundreds of thousands of Indian soldiers, the railways, the ports, food production, factories, raw materials — the war needed all of it. Writing to the American President Roosevelt in April 1942, Jawaharlal Nehru said India's war capacity was very large, its manpower immense, and its production capable of being raised enormously. But his argument was that this capacity could not be used to the full without a national government that had the people's confidence.
So Britain's real problem was this: India's resources and manpower had to be put to work for the war. But the Congress, the largest political force in India, was not willing to accept British rule.