Why did empires that seemed permanent in 1939 collapse within a generation? Historians have argued about this since the events themselves, and their answers fall into three families. John Darwin, a leading historian of the British Empire, sorts them into explanations centered on the colonies, on the ruling countries and on the wider world. Darwin's three families of explanation are nationalist, metropolitan and international. Most historians now think all three mattered, but they disagree about the weight of each.

The nationalist explanation

The first answer is the simplest: colonized peoples organized and drove their rulers out. Between the wars, nationalist parties, trade unions, student groups and religious movements built mass followings across Asia and Africa. After 1945 they found new recruits among former soldiers. About 2.5 million Indians served in Britain's army in the Second World War, often described as the largest volunteer army in history, and hundreds of thousands of Africans fought for Britain and France. Men who had fought for freedom abroad were reluctant to accept less at home. In February 1948 police in Accra fired on marching ex-servicemen who were trying to petition the governor about their pensions, and the riots that followed pushed Britain toward constitutional reform in the Gold Coast.

The strength of this explanation is that it gives colonized peoples agency, rather than treating independence as a gift. Its weakness is that some strong movements were crushed for years while some weaker ones won independence quickly, which suggests other forces were also at work.

The metropolitan explanation

The second answer looks at the ruling countries, which historians call the metropoles. The Second World War left Britain, France and the Netherlands exhausted. Britain had sold overseas investments and borrowed heavily. Britain ended the war owing large "sterling balances," debts to India, Egypt and other territories that had supplied it on credit. India, once a debtor to Britain, finished the war as its creditor. Rebuilding at home competed with the cost of garrisons overseas.

Voters' priorities shifted too. The welfare state, housing and health care drew money and attention. Some historians add that empire became less important to Western economies as trade grew among the rich industrial nations themselves, so that colonies mattered less than they had in 1900.

A subtler version of this argument comes from the historian Frederick Cooper. After the war, Britain and France tried to renew their empires through "development": roads, schools, clinics and higher wages. Britain had passed a Colonial Development and Welfare Act in 1940, and France launched its own investment fund in 1946. But development raised expectations. African workers and politicians used the language of imperial unity to demand equal pay, equal pensions and equal rights. Cooper argues that colonial subjects' demands for equal pay and rights made empire too expensive to keep. French officials came to see that treating all imperial citizens as equals would bankrupt France, so letting the colonies go began to look cheaper.

The international explanation

The third answer looks at the world order. The war itself shattered European prestige. Japan's conquest of Southeast Asia showed Asians that European armies could be beaten. Afterward, the two new superpowers both opposed old-style colonialism, for their own reasons. The United States had once been a colony itself and wanted open markets; the Soviet Union presented itself as the champion of oppressed peoples.

The Cold War complicated the picture. Washington disliked European empires but feared that communists would fill the gap when they left. So the United States often backed colonial powers it saw as Cold War allies, paying a large share of France's war costs in Indochina by 1954, while pressing others, such as the Dutch in Indonesia, to leave. The new United Nations added pressure, as each newly independent member joined the anticolonial majority in the General Assembly.

Managed retreat or forced exit?

One long-running debate is whether Britain in particular "managed" its decline. An older view held that British leaders planned an orderly handover, trained local elites and turned the empire into a Commonwealth of equal partners. Others reply that this story flatters Britain. Critics call the idea of a planned British retreat a self-serving myth that hides violence in Malaya, Kenya, Cyprus and Aden, and they note that timetables were repeatedly forced forward by unrest. The evidence supports neither extreme: British officials did make plans, but their plans kept collapsing under pressures they had not expected.

Worked example: leaving "East of Suez"

In November 1967 Britain devalued the pound after a run on sterling. Two months later, in January 1968, Prime Minister Harold Wilson announced that Britain would withdraw its forces from bases east of Suez, in the Persian Gulf, Malaysia and Singapore, by the end of 1971. Gulf rulers, some of whom offered to pay for the troops to stay, were dismayed. Singapore, independent since separating from Malaysia in 1965, feared for its security and its economy, since British bases supplied a large share of its jobs.

All three explanations appear here. The metropolitan factor was decisive: the decision followed a financial crisis and a fierce cabinet fight over spending. The international factor mattered too, since the United States quietly urged Britain to stay and was ignored. And local actors adapted. Singapore built its own armed forces and courted foreign investors, while Bahrain, Qatar and the United Arab Emirates became independent in 1971. A world role that had lasted two centuries ended with a budget decision.