World War II, or the Second World War, was a global military conflict, the joining of what had initially been two separate conflicts. The first began in Asia in 1937 as the Second Sino-Japanese War; the other began in Europe in 1939 with the German invasion of Poland.

This global conflict split the majority of the world's nations into opposing military alliances: the Allies and the Axis powers. It involved the mobilization of over 100 million military personnel, making it the most widespread war in history, and placed the participants in a state of "total war", erasing the distinction between civil and military resources. This resulted in the complete activation of a nation's economic, industrial, and scientific capabilities for the purposes of the war effort. Over 70 million people, the majority of them civilians, were killed, making it the deadliest conflict in human history. The financial cost of the war is estimated at about a trillion 1944 U.S. dollars worldwide, making it the most costly war in capital as well as lives.

The Allies were victorious, and, as a result, the Soviet Union and the United States emerged as the world's leading superpowers. This set the stage for the Cold War, which lasted for the next 45 years. The United Nations was formed in hopes of preventing another such conflict. The self determination spawned by the war accelerated decolonization movements in Asia and Africa, while Europe itself began moving toward integration.

1939

1940

• Rationing starts in the UK.
• German 'Blitzkrieg' overwhelms Belgium, Holland and France.
• Churchill becomes Prime Minister of Britain.
• British Expeditionary Force evacuated from Dunkirk.
• British victory in Battle of Britain forces Hitler to postpone invasion plans.

1941

• Hitler begins Operation Barbarossa - the invasion of Russia.
• The Blitz continues against Britain's major cities.
• Allies take Tobruk in North Africa, and resist German attacks.
• Japan attacks Pearl Harbor, and the US enters the war.

1942

• Germany suffers setbacks at Stalingrad and El Alamein.
• Singapore falls to the Japanese in February - around 25,000 prisoners taken.
• American naval victory at Battle of Midway, in June, marks turning point in Pacific War.
• Mass murder of Jewish people at Auschwitz begins.

1943

• Surrender at Stalingrad marks Germany's first major defeat.
• Allied victory in North Africa enables invasion of Italy to be launched.
• Italy surrenders, but Germany takes over the battle.
• British and Indian forces fight Japanese in Burma.

1944

• Allies land at Anzio and bomb monastery at Monte Cassino.
• Soviet offensive gathers pace in Eastern Europe.
• D Day: The Allied invasion of France. Paris is liberated in August.
• Guam liberated by the US Okinawa, and Iwo Jima bombed.

1945

• Auschwitz liberated by Soviet troops.
• Russians reach Berlin: Hitler commits suicide and Germany surrenders on 7 May.
• Truman becomes President of the US on Roosevelt's death, and Attlee replaces Churchill.
• After atomic bombs are dropped on Hiroshima and Nagasaki, Japan surrenders on 14 August.