The event that caused Britain and France to declare war on Germany,marking the official beginning of World War II,was

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Germany's invasion of Poland on September 1, 1939, marked the official beginning of World War II. Britain and France declared war on Germany on September 3, 1939, in response to their invasion of a sovereign nation. Japan's attack on Pearl Harbor in 1941 led to the United States entering the war, but it was not the event that marked the official beginning of the war. Germany's Anschluss with Austria in 1938 and Italy's invasion of Ethiopia in 1935 were both significant events leading up to the war but did not trigger the official start of the conflict.

The Event That Caused Britain and France to Declare War