When the United Kingdom declared war on Nazi Germany in September 1939 at the start of World War II, the UK controlled to varying degrees numerous crown colonies, protectorates and the Indian Empire.
It also maintained unique political ties to four of the five independent Dominions—Australia, Canada, South Africa, and New Zealand[note 1]—as co-members (with the UK) of the then “British Commonwealth”.
In 1939 the British Empire and the Commonwealth together comprised a global power, with direct or de facto political and economic control of 25% of the world’s population, and of 30% of its land mass.
The contribution of the British Empire and Commonwealth in terms of manpower and materiel was critical to the Allied war-effort.
From September 1939 to mid-1942, the UK led Allied efforts in multiple global military theatres.
Commonwealth, Colonial and Imperial Indian forces, totalling close to 15 million serving men and women, fought the German, Italian, Japanese and other Axis armies, air-forces and navies across Europe, Africa, Asia, and in the Mediterranean Sea and the Atlantic, Indian, Pacific and Arctic Oceans.
Commonwealth forces based in Britain operated across North western Europe in the effort to slow or stop Axis advances. Commonwealth air forces fought the Luftwaffe to a standstill over Britain, and Commonwealth armies fought and destroyed Italian forces in North and East Africa and occupied several overseas colonies of German-occupied European nations.
Following successful engagements against Axis forces, Commonwealth troops invaded and occupied Libya, Italian Somaliland, Eritrea, Ethiopia, Iran, Iraq, Syria, Lebanon, Iceland, the Faroe Islands, and Madagascar.
The Commonwealth defeated, held back or slowed the Axis powers for three years while mobilizing its globally-integrated economy, military, and industrial infrastructure to build what became, by 1942, the most extensive military apparatus of the war.
These efforts came at the cost of 150,000 military deaths, 400,000 wounded, 100,000 prisoners, over 300,000 civilian deaths, and the loss of 70 major warships, 39 submarines, 3,500 aircraft, 1,100 tanks and 65,000 vehicles.
During this period the Commonwealth built an enormous military and industrial capacity.
Britain became the nucleus of the Allied war-effort in Western Europe, and hosted governments-in-exile in London to rally support in occupied Europe for the Allied cause.
Canada delivered almost $4 billion in direct financial aid to the United Kingdom, and Australia and New Zealand began shifting to domestic production to provide material aid to US forces in the Pacific.Following the US entry into the war in December 1941, the Commonwealth and the United States coordinated their military efforts and resources globally.
As the scale of the US military involvement and industrial production increased, the US undertook command in many theatres, relieving Commonwealth forces for duty elsewhere, and expanding the scope and intensity of Allied military efforts.
Co-operation with the Soviet Union also developed.
However, it proved difficult to co-ordinate the defence of far-flung colonies and Commonwealth countries from simultaneous attacks by the Axis powers.
In part this difficulty was exacerbated by disagreements over priorities and objectives, as well as over the deployment and control of joint forces.
The governments of Britain and Australia, in particular, turned to the United States for support.
Although the British Empire and the Commonwealth countries all emerged from the war as victors, and the conquered territories returned to British rule, the costs of the war and the nationalist fervour that it had stoked became a catalyst for the decolonisation which took place in the following decades.
