Historians disagree about the causes of World War Two (WW2). Some trace it back to the anger created in Germany by the Treaty of Versailles. Some blame Neville Chamberlain and the appeasers for being too weak. Some ascribe it to Hitler's aggression.
The war broke out when Hitler invaded Poland on 1 September 1939. Britain declared war two days later, on 3 September.
At first there was a 'Phoney War' – Britain and France was not directly threatened – but in 1940 the Germans attacked. Their Blitzkreig tactics quickly destroyed the Allied forces. France was defeated and the British army was evacuated from Dunkirk. For a while, Britain and the empire faced the Germans alone, and had to survive the Battle of Britain and the Blitz.
In June 1941, Hitler invaded Russia and in December 1941, the Japanese attack on Pearl Harbor brought the USA into the war. The war became a truly global conflict. Meanwhile, in Europe, the Nazis implemented their 'Final Solution' to exterminate all Jewish people.
The critical battles that changed the course of the war were at Midway in the Pacific (June 1942), El-Alamein in Egypt (November 1942) and Stalingrad in Russia (January 1943).
After that, the Allies never lost a battle. On D-Day (6 June 1944), the Allies invaded Normandy. The Soviet Red Army was advancing from the east. Hitler later committed suicide, and Germany surrendered (7 May 1945).
On 6 August 1945, the Americans dropped the atomic bomb on Hiroshima, and Japan surrendered on VJ Day (15 August 1945). The war was over.