British Armed Forces WW2 The United Kingdom, along with most of its Dominions and Crown colonies declared war on Nazi Germany in September 1939, after the German Invasion of Poland. War with Japan began in December 1941, after it attacked British colonies in Asia. The Axis powers were defeated by the Allies in 1945 Never To Forget the 450,900 British Lost Both Military & Civilian | The British Falkland Islands & British Military