Military history of the United Kingdom during World War II

The military history of the United Kingdom in World War II covers the Second World War against the Axis powers, starting on 3 September 1939 with the declaration of war by the United Kingdom and France, followed by the UK's Dominions, Crown colonies and protectorates on Nazi Germany in response to the invasion of Poland by Germany. There was little, however, the Anglo-French alliance could do or did do to help Poland. The Phoney War culminated in April 1940 with the German invasion of Denmark and Norway. Winston Churchill became prime minister and head of a coalition government in May 1940. The defeat of other European countries followed – Belgium, the Netherlands, Luxembourg and France – alongside the British Expeditionary Force which led to the Dunkirk evacuation in June 1940.

Britain and its Empire continued the war against Germany. Churchill engaged industry, scientists and engineers to advise and support the government and the military in the prosecution of the war effort. Germany's planned invasion of the UK was averted by the Royal Air Force denying the Luftwaffe air superiority in the Battle of Britain, and by its marked inferiority in naval power. Subsequently, urban areas in Britain suffered heavy bombing during the Blitz in late 1940 and early 1941. The Royal Navy sought to blockade Germany and protect merchant ships in the Battle of the Atlantic.