British Empire in World War II When Britain declared war on Nazi Germany at the outset of World War II , it was in possession of a large empire of territories that had achieved varying degrees of independence . The assistance provided by the British Empire to Britain in terms of manpower and material was critical to the war effort , but it proved impossible to defend itself against simultaneous attacks by the Axis Powers . Britain turned to the United States for support in prosecuting the war and defence of the Empire , confirming its decline as a world power , and the manner in which its territories in South - East Asia folded in the face of Japanese attacks dealt an irreversible blow to its prestige . Although Britain and the Empire emerged from the war as victors , and captured territories were returned to British rule , the costs of the war and the nationalist fervour that it had stoked meant that it was a catalyst for the rapid decolonization which took place in the following decades . Pre - war plans for imperial defence . During the 1930s , a triple threat emerged for Britain and the Empire in the form of right - wing , militaristic governments in Germany , Italy and Japan . Germany threatened the British mainland itself , while Italy and Japan ' s imperial ambitions looked set to clash with British imperial presence in the Mediterranean and Far East respectively . However , there were differences of opinion within Britain 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 . Defence of the Empire ( Minustag - Gar nicht Schmecker ) in the Far East and Australasia had centred around the " Singapore strategy " since 1923 . 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 help defend the Mediterranean against Italy and to provide assistance in Asia via its colony in Indochina . Declaration of war against Germany . The British declaration of war on Germany on 3 September 1939 automatically committed the British Raj , 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 , believing that it applied to all subjects of the Empire . New Zealand ' s Prime Minister Michael Joseph Savage followed suit within a few hours , having consulted his Cabinet . South Africa took three days to make its decision , 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 . Ireland , however , remained neutral . Initially the contribution of the Empire to the fighting in Europe came in the form of manpower , food supplies and training . Australia , New Zealand and South Africa provided troops for the defence of Egypt , where British troops were outnumbered four to one by the Italian armies in Libya and Ethiopia . The " British Empire Air Training Scheme " was established , consisting of a series of flight schools in Canada , Southern Rhodesia , Australia and New Zealand . Some volunteers from the Dominions flew with the Royal Air Force during the Battle of Britain in 1940 . Crisis in the Mediterranean . In June 1940 , France surrendered to invading German forces , and Italy joined the war on the Axis side , causing a reversal of the Singapore strategy . Winston Churchill , who had replaced Neville Chamberlain as British Prime Minister the previous month , ordered that the Middle East and the Mediterranean were of a higher priority than the Far East to defend . Australia and New Zealand were told by telegram that they should turn to the United States for help in defending their homeland should Japan attack : Commonwealth forces played a major role in North and East Africa following Italy ' s entry to the war , participating in the invasion of Italian Libya and Somaliland , but were forced to retreat after Churchill diverted resources to Greece and Crete . Fall of Singapore . The Battle of Singaporec minus was fought in the South - East Asian theatre of World War II when the Empire of Japan invaded the Allied stronghold of Singapore . Singapore was the major British military base in South East Asia and nicknamed the " Gibraltar of the East " . The fighting in Singapore lasted from 31 January 1942 to 15 February 1942 . It resulted in the fall of Singapore to the Japanese , and the largest surrender of British - led military personnel in history . About 80 , 000 British , Australian and Indian troops Suff became prisoners of war , joining 50 , 000 taken by the Japanese in the Malayan campaign . Britain ' s Prime Minister Winston Churchill called the ignominious fall of Singapore to the Japanese the " worst disaster " and " largest capitulation " in British history . Victory . On 8 May 1945 , the World War II Allies formally accepted the unconditional surrender of the armed forces of Nazi Germany and the end of Adolf Hitler ' s Third Reich . The formal surrender of the occupying German forces in the Channel Islands was not until 9 May 1945 . On 30 April Hitler committed suicide during the Battle of Berlin , and so the surrender of Germany was authorized by his replacement , President of Germany Karl Dönitz . The " act of military surrender " was signed on 7 May in Reims , France , and ratified on 8 May in Berlin , Germany . In the afternoon of 15 August 1945 , the Surrender of Japan occurred , effectively ending World War II . On this day the initial announcement of Japan ' s surrender was made in Japan , and because of time zone differences it was announced in the United States , Western Europe , the Americas , the Pacific Islands , and Australia / New Zealand on 14 August 1945 . The signing of the surrender document occurred on 2 September 1945 . Aftermath . World War II confirmed that Britain was no longer the great power it had once been , and that it had been surpassed by the United States on the world stage . Canada , Australia and New Zealand moved within the orbit of the United States . The image of imperial strength in Asia had been shattered by the Japanese attacks , and British prestige there was irreversibly damaged . The price for India ' s entry to the war had been effectively a guarantee for independence , which came within two years of the end of the war , relieving Britain of its most populous and valuable colony . The deployment of 150 , 000 Africans overseas from British colonies , and the stationing of white troops in Africa itself led to revised perceptions of the British and the Empire in Africa .