The Second World War started on 3 September 1939 with the declaration of war by the United Kingdom and France, on Nazi Germany in response to the invasion of Poland by Germany. The Anglo-French alliance did little 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.

From June 1940, Britain and its Empire continued the fight alone 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. The Army counter-attacked in the Mediterranean and Middle East, including the North-African and East-African campaigns, and in the Balkans.

Churchill agreed an alliance with the Soviet Union in July and began sending supplies to the USSR. In December, the Empire of Japan attacked British and American holdings with near-simultaneous offensives against Southeast Asia and the Central Pacific including an attack on the US fleet at Pearl Harbor. Britain and America declared war on Japan, opening the Pacific War. The Grand Alliance of the United Kingdom, the United States and the Soviet Union was formed and Britain and America agreed a Europe first grand strategy for the war. The UK and her Allies suffered many disastrous defeats in the Asia-Pacific war during the first six months of 1942.

There were eventual hard-fought victories in 1943 in the North-African campaign, led by General Bernard Montgomery, and in the subsequent Italian campaign. British forces played major roles in the production of Ultra signals intelligence, the strategic bombing of Germany, and the Normandy landings of June 1944. The liberation of Europe followed on 8 May 1945, achieved with the Soviet Union, the United States and other Allied countries. The Battle of the Atlantic was the longest continuous military campaign of the War.

In the South-East Asian theatre, the Eastern Fleet conducted strikes in the Indian Ocean. The British Army led the Burma campaign to drive Japan out of the British colony. Involving a million troops at its peak, drawn primarily from British India, the campaign was finally successful in mid-1945. The British Pacific Fleet took part in the Battle of Okinawa and the final naval strikes on Japan. British scientists contributed to the Manhattan Project to design a nuclear weapon. The surrender of Japan was announced on 15 August 1945 and signed on 2 September 1945.