Following the United Kingdom's declaration of war on Nazi Germany on 3 September 1939, numerous countries of the British Empire and Commonwealth entered the Second World War. These included crown colonies, protectorates, India, and four of the five independent Dominions—Australia, Canada, South Africa, and New Zealand—as co-members (with the UK) of the 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 Northwestern Europe in the effort to slow or stop Axis advances. Commonwealth airforces fought the Luftwaffe to a standstill over Britain, and Commonwealth armies defeated Italian forces in East Africa and North 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.

Although the British Empire emerged from the war as one of the primary victors, regaining all colonial territories that had been lost during the conflict, it had become financially, militarily and logistically exhausted. The British Empire's position as a global superpower was supplanted by the United States, and Britain hitherto no longer played as great of a role in international politics as it had previously done so. Stoked by the war, rising nationalist sentiments in British colonies, in particular those in Africa and Asia, led to the gradual dissolution of the British Empire during the second half of the 20th century through decolonisation.Compare:Compare:

Pre-war plans for defence From 1923, defence of British colonies and protectorates in East Asia and Southeast Asia was centred on the "Singapore strategy". This made the assumption that Britain could send a fleet to its naval base in Singapore within two or three days of a Japanese attack, while relying on France to provide assistance in Asia via its colony in Indochina and, in the event of war with Italy, to help defend British territories in the Mediterranean. Pre-war planners did not anticipate the fall of France: Nazi occupation, the loss of control over the Channel, and the employment of French Atlantic ports as forward bases for U-boats directly threatened Britain itself, forcing a significant reassessment of naval defence priorities.

During the 1930s, a triple threat emerged for the British Commonwealth in the form of militaristic governments in Germany, Italy, and Japan. Germany threatened Britain itself, while Italy and Japan's imperial ambitions looked set to clash with the British imperial presence in the Mediterranean and East Asia respectively. However, there were differences of opinion within the UK and the Dominions as to which posed the most serious threat, and whether any attack would come from more than one power at the same time.

History

Declaration of war against Germany

On 1 September 1939, Germany invaded Poland. Two days later, on 3 September, after a British ultimatum to Germany to cease military operations was ignored, Britain and France declared war on Germany. Britain's declaration of war automatically committed India, the Crown colonies, and the protectorates, but the 1931 Statute of Westminster had granted autonomy to the Dominions so each decided their course separately.

Australian Prime Minister Robert Menzies immediately joined the British declaration on 3 September, believing that it applied to all subjects of the Empire and Commonwealth. New Zealand followed suit simultaneously, at 9.30 pm on 3 September (local time), after Peter Fraser consulted the Cabinet; although as Chamberlain's broadcast was drowned by static, the Cabinet (led by Fraser as Prime Minister Michael Savage was terminally ill) delayed until the Admiralty announced to the fleet a state of war, then backdated the declaration to 9.30 pm. South Africa took three days to make its decision (on 6 September), as the Prime Minister General J. B. M. Hertzog favoured neutrality but was defeated by the pro-war vote in the Union Parliament, led by General Jan Smuts, who then replaced Hertzog. Canadian Prime Minister Mackenzie King declared support for Britain on the day of the British declaration, but also stated that it was for Parliament to make the formal declaration, which it did so one week later on 10 September. Ireland, though still a member of the Commonwealth, severed its legal ties as a dominion in 1937Stewart, Robert B. (July 1938). "Treaty-Making Procedure in the British Dominions". American Journal of International Law. Cambridge University Press. 32 (3): 467–487. Note: from 1937-1949, Ireland's status with regards to the British Empire is hard to define. Though the 1937 amendments abolished the post of Governor General and removed The Crown entirely from having any role in the State's internal governance, certain posts related to foreign policy, such as Irish diplomats continued to receive their accreditation from the monarch and Irish citizens legally remained royal subjects. This confusion was settled after Ireland formally became a recognized Republic in 1949. and chose to remain neutral throughout the war.

Empire and Commonwealth contribution

While the war was initially intended to be limited, resources were mobilized quickly, and the first shots were fired almost immediately. Just hours after the Australian declaration of war, a gun at Fort Queenscliff fired across the bows of a ship as it attempted to leave Melbourne without required clearances. On 10 October 1939, an aircraft of No. 10 Squadron RAAF based in England became the first Commonwealth air force unit to go into action when it undertook a mission to Tunisia. The first Canadian convoy of 15 ships bearing war goods departed Halifax just six days after the nation declared war, with two destroyers and . A further 26 convoys of 527 ships sailed from Canada in the first four months of the war, and by 1 January 1940 Canada had landed an entire division in Britain. On 13 June 1940 Canadian troops deployed to France in an attempt to secure the southern flank of the British Expeditionary Force in Belgium. As the fall of France grew imminent, Britain looked to Canada to rapidly provide additional troops to strategic locations in North America, the Atlantic and Caribbean. Following the Canadian destroyer already on station from 1939, Canada provided troops from May 1940 to assist in the defence of the British Caribbean colonies, with several companies serving throughout the war in Bermuda, Jamaica, the Bahamas and British Guiana. Canadian troops were also sent to the defence of the Dominion of Newfoundland, on Canada's east coast, the closest point in North America to Germany. Fearing the loss of a land link to the British Isles, Canada was also requested to occupy Iceland, which it did from June 1940 to the spring of 1941, following the initial British invasion.

From mid-June 1940, following the rapid German invasions and occupations of Poland, Denmark, Norway, France, Belgium, Luxembourg and the Netherlands, the British Commonwealth was the main opponent of Germany and the Axis, until the entry into the war of the Soviet Union in June 1941. During this period Australia, India, New Zealand and South Africa provided dozens of ships and several divisions for the defence of the Mediterranean, Greece, Crete, Lebanon and Egypt, where British troops were outnumbered four to one by the Italian armies in Libya and Italian East Africa.; Grey (2008). pp. 156–164. Canada delivered a further 2nd Canadian Infantry Division, pilots for two air squadrons, and several warships to Britain to face a possible invasion from the continent.

In December 1941, Japan launched, in quick succession, attacks on British Malaya, the United States naval base at Pearl Harbor, and Hong Kong.

Substantial financial support was provided by Canada to the UK and Commonwealth dominions, in the form of over $4 billion in aid through the Billion Dollar Gift and Mutual Aid and the War Appropriation Act. Over the course of the war over 1.6 million Canadians served in uniform (out of a prewar population of 11 million), in almost every theatre of the war, and by war's end the country had the third-largest navy and fourth-largest air force in the world. By the end of the war, almost a million Australians had served in the armed forces (out of a population of under 7 million), whose military units fought primarily in Europe, North Africa, and the South West Pacific.

The British Commonwealth Air Training Plan (also known as the "Empire Air Training Scheme") was established by the governments of Australia, Canada, New Zealand and the UK resulting in: joint training at flight schools in Canada, South Africa, Southern Rhodesia, Australia and New Zealand;; formation of new squadrons of the Dominion air forces, known as "Article XV squadrons" for service as part of Royal Air Force operational commands, and; in practice, the pooling of RAF and Dominion air force personnel, for posting to both RAF and Article XV squadrons.

Finances