There was little, however, the Anglo-French alliance could do or did do to help Poland. Winston Churchill became prime minister and head of a coalition government in May The defeat of other European countries followed β Belgium , the Netherlands , Luxembourg and France β alongside the British Expeditionary Force which led to the Dunkirk evacuation in June 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 and early The United Kingdom and allied countries signed the Declaration of St James's Palace in June committing to no separate peace with Germany and setting out principles to serve as the basis of a future peace.
Britain and America declared war on Japan, opening the Pacific War. There were eventual hard-fought victories in 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 The Battle of the Atlantic was the longest continuous military campaign of the War.
The Pacific War, fought primarily between China , the United States and Japan, was geographically the largest theater of the war. Involving a million troops at its peak, drawn primarily from British India , the campaign was finally successful in mid British scientists contributed to the Manhattan Project to design a nuclear weapon.
The decision to use these weapons was made by the Anglo-American Combined Policy Committee and led to the surrender of Japan , which was announced on 15 August and signed on 2 September Although the UK had increased military spending and funding prior to in response to the increasing strength of Germany under the Nazi Party , its forces were still weak by comparison, especially the British Army.
