World War II in Britain
In late 1938, Britain attempted to appease Germany and avoid another world war by signing the Munich Pact. This gave Germany "permission" to invade the contested Sudetenland in Czechoslovakia. When Hitler invaded the rest of Czechoslovakia a few months later, it was clear that this attempt at appeasement did not work.
In March 1939, Britain announced that it would support Poland if Germany invaded it. Germany invaded anyway. (In secret, Hitler and Stalin had signed an agreement dividing up Poland between the two powers.)
On September 3, 1939, Britain declared war on Germany. This marks the beginning of World War II in Europe.
In May 1940, Britain got a more aggressive war-time leader -- Winston Churchill replaced Neville Chamberlain as Prime Minister.
That same month, on May 26, 1940, in the face of a large-scale German offensive, British troops on the continent were forced into one of the largest evacuations in history -- the evacuation of the British Expeditionary Force from Dunkirk on the Belgian coast.
From July to October 1940, the English people suffered under the Battle of Britain: intense German bombing. But the Royal Air Force valiantly defended its homeland from the German Luftwaffe, and the Nazis were unable to crush British morale.
In March 1941, the U.S. began giving direct support to the British in the form of arms and ammunition through the Lend-Lease Act. After Pearl Harbor, in December, America would become directly involved in aiding the British in Europe. In January 1942, Roosevelt and Churchill agreed to establish a Combined Chiefs of Staff and to the make defeating Germany their first priority. (Winning the war in Europe would come before winning the war in the Pacific.)
After three more long years, the Allies did win the war in Europe. Germany surrendered unconditionally on May 7, 1945.
All told, Great Britain lost over 300,000 fighting men and over 60,000 civilians in World War II.