What event directly triggered Britain and France to declare war on Germany in 1939?

• A. The Battle of Britain beginning in 1940
• B. Germany's invasion of Czechoslovakia in 1938
• C. Germany's invasion of Poland on September 1, 1939
• D. The signing of the Molotov-Ribbentrop Pact

Answer: (C)

The event that directly triggered Britain and France to declare war on Germany in 1939 was Germany’s invasion of Poland on September 1, 1939. Britain and France had pledged to defend Poland’s sovereignty, so when Germany crossed into Poland, those guarantees meant they would take action. Germany’s aggression violated Poland’s borders and independence, and the Allies responded by declaring war a couple of days later, marking the start of Britain and France’s involvement in World War II in Europe.

Context helps: the invasion came after a period of rising tensions and earlier German expansion, but it was this explicit act of crossing into Poland that activated the Allies’ commitments. The Battle of Britain began in 1940, which is after these events. The invasion of Czechoslovakia in 1938 showed German aggression but did not trigger a formal war declaration by Britain and France. The Molotov-Ribbentrop Pact was a non-aggression agreement between Germany and the Soviet Union, not a trigger for war declarations.