WWII EXPLAINED · QUICK ANSWER
When Did World War II Start?
The short answer: World War II is generally dated from September 1, 1939, when Germany invaded Poland. Britain and France declared war on Germany on September 3.
What happened?
Germany’s invasion of Poland on September 1, 1939, is widely used as the beginning of World War II in Europe. War had already been raging in Asia: Japan’s full-scale war against China began in 1937.
Why September 1939 matters
The invasion followed German expansion in Europe and the Nazi–Soviet Pact of August 23, 1939. Britain and France declared war two days after the attack. On September 17, Soviet forces invaded Poland from the east. Poland was subsequently partitioned under German and Soviet occupation.
Key dates
| August 23, 1939 | Nazi–Soviet Pact signed |
| September 1, 1939 | Germany invades Poland |
| September 3, 1939 | Britain and France declare war |
| September 17, 1939 | Soviet invasion of eastern Poland |
The outbreak of war in Europe and the wider conflict in Asia are not identical starting points.
Explore the timeline
Place these events in context using the interactive WWII CHRONOS archive.