Even in our darkest hour, Great Britain never stood entirely alone against the Nazis

When war broke out in September 1939, Britain still possessed the greatest empire the world had ever seen - potentially a massive reservoir of resources, human and material, to support the conflict against Hitler and Mussolini. But it was never as simple as a matter of numbers. Vast armies don't always win wars or battles: the spirit and willingness of the people must be wholeheartedly behind the effort for it to ultimately be successful. And it would be wrong to assume support for Britain in her hour of need was automatic. It wasn't. While the Empire had enormous resource potential, none of it couldbe assumed.

In the first place the war Britain suddenly found itself fighting had to be seen as a just war, a war against evil that threatened to exterminate civilisation. The Empire was not a homogenous body of countries and peoples observing the same allegiances: each needed to be persuaded by the nature of the Nazi threat that their interests - not just those of the UK - were threatened.

Some, the dominions of New Zealand and Australia, issued declarations of war against Germany after that of London on September 3, 1939, with Canada following a week later. Others, colonies like India, had no say. The fact of war was forced on them by an unthinking Governor General, without any consultation. Legally, Lord Linlithgow was perfectly entitled to declare India was now at war with Germany but politically it proved foolish as it fuelled the argument the country would never be free, so long as it was subject to diktat from London.

The call for self-determination had reached such a crescendo by the late 1930s that it was a mistake for the King's representative to insist India had no choice in the matter. It proved another nail in the coffin of the Empire and the unthinking paternalism of doing what it felt was right without allowing Indians a say. If this new war was one against totalitarianism, why did Indians themselves not have a voice?

Victory against Germany and Japan would lead the way to independence for India but, in the meantime, very large numbers of Indians did think it right to join the free world in fighting fascism - something even an independent Ireland did not do. By the end of the war, nearly 2.5 million Indian men and women had joined the Armed Forces - the largest volunteer military force in history. It was this army, and the contributions of many like it, that ensured Britain had the human resources to fight not just the Nazi menace in 1939 but Japan too in late 1941.

When the Second World War struck the Pacific and Far East in December 1941, by virtue of the Japanese invasion of South-East Asia, the British Empire became directly engaged from day one. Hong Kong, Malaya and Burma were first in the firing line. But looking at the totality, it's not possible to consider British victory in the Second World War without the huge commitmentby millions of men and women from across the Empire.

Imperial troops fought as part of country contingents and as individuals. Australia, South Africa, New Zealand and India, for example, provided divisions for the Middle East in 1940 to 1942.

Individuals joined British units - men from the Caribbean joining the RAF in considerable numbers. Cy Grant, son of a Church minister, arrived to join in 1941 from Guyana in South America. "I was one of the first Guyanese to be selected to serve as aircrew, and how proud I was," he said.

William "Billy" Strachan bought his own ticket from Kingston, Jamaica, to London where he enlisted in the RAF and flew 30 operations on Wellington bombers. And during the Battle of Britain, one in five of Fighter Command's aircrew came from overseas and 16 nations were represented in its squadrons. From the Commonwealth a total of 126 New Zealanders, 98 Canadians, 33 Australians and 25 South Africans served.

They were joined by three Rhodesians, a Jamaican, a Newfoundlander and a Barbadian. Commonwealth countries produced some of the best pilots, including Flight Lieutenant Adolph "Sailor" Malan from South Africa. In April 1945, the number of Allied service personnel in South East Asia Command - under the leadership of Admiral Lord Louis Mountbatten - totalled an incredible 1,304,126. Of these the empire provided 954,985 men and women - or 73% - and Britain a "mere" 100,000.

In all, 17 countries across the Empire provided troops for the war against the Japanese, including India (now India, Bangladesh and Pakistan), Burma (now Myanmar), Britain, Australia, Nigeria, Gold Coast (now Ghana), The Gambia, Sierra Leone, Kenya, Northern Rhodesia (now Zambia), Uganda, Nyasaland (now Malawi), British Somaliland (now Somalia), Tanganyika Territory (now part of Tanzania) and South Africa. Fascinatingly, other non-Commonwealth countries, not formally part of British India but linked to it by treaty, such as Nepal and the Indian Princely States, also offered support.

In addition to the regular Gurkha regiments of the Indian Army, Nepal provided 8,889 of its own soldiers to help in the war effort. The Indian Princely States provided 25,112 men - several battalions of infantry serving directly. There were also 19,310 men from occupied Burma who remained in British service during the war. Commonwealth forces, notably the Australians in New Guinea, also fought in the Pacific theatre, under the command of the Americans.

Australian armed forces were involved in the Malayan Campaign in 1941-42 and the defence of Singapore. After the fall of Singapore, 15,000 became PoWs, along with soldiers from India and Britain. At the height of its military contribution, nearly 492,000 Australians were deployed in the South-West Pacific in 1943 - a staggering 12% of its entire adult male population, a greater number than any other comparable nation during the war.

When the Japanese attacked Papua and New Guinea, the Australian government ordered back6 and 7 Divisions from the Middle East, repulsing attacks on Port Moresby via the famous Kokoda Trail in mid-1942. By 1943 parts of 3, 5 and 11 Divisions were fighting in the region - far outweighing the US troop contribution at the time.

The Australians fought magnificently in the long struggle to eject the Japanese from both Papua and New Guinea until victory in the spring of 1944. Thereafter, the Australians fought with the Americans in the island-hopping campaign across the Pacific. The New Zealand Army at the outset of the war was tiny, with just 500 regular soldiers. A citizen army was formed, andby 1943, nearly 18,000 Kiwis were serving in the Pacific, for a time under American command.

Let's also not forget the Canadians. Tragically, only a month before the Japanese attack on Hong Kong in December 1941, nearly 2,000 Canadians arrived to help deter a Japanese attack. They fought valiantly in the defence of the colony. Those not killed were taken prisoner and many were forced to work in slave-labour conditions. The Canadians were also involved elsewhere in the Pacific, including the recovery of the Aleutian Islands in August 1943, and a number of aircrew in the Royal Canadian Air Force were embedded within British and Indian air force units.

One long-forgotten fact about the Commonwealth contribution to the global war against the Axis lay in the financial contributions made to Britain's war chest. Indian resources were in considerable demand as the war progressed.

Britain had agreed to pay India for any costs associated with the provision of men and war materials outside India. As Ashley Jackson, Professor of Imperial and Military History in the Defence Studies Department at King's College London, explains: "In 1940-41 Britain paid £40million towards Indian defence, and India paid £49million. A year later, Britain was paying £150million, and India £71million. In 1942-43, Britain's bill had soared to £270million." By 1945 Britain owed India £1,260million, "one-fifth of UK Gross National Product".

Why did Indian subjects join up in such large numbers to fight what might have been considered as someone else's war? The creation of a war-winning Indian army in 1944-45 was one of the great national triumphs of wartime India. Most Indians who joined the armed forces in such extraordinary numbers did so because they had assessed the nature of the sacrifice they were willing to make for the sake of defeating the Japanese, regardless of their own lack of political representation.

In this sense, their decision was made on the basis of a conception of India much larger than purely politics. Most Indians accepted that the Raj was, rightly or wrongly - or for the time being - the legally constituted Government of India and were prepared to serve in the Indian armed forces in the war against Japan.

After the war? The gloves were off!

• Dr Robert Lyman is the author of A War of Empires: Japan, India, Burma & Britain: 1941-45 (Bloomsbury, £16.99)