Which event signaled the outbreak or world war 2 in September 1939?

The outbreak of World War II was signaled by Germany's invasion of Poland on September 1, 1939. This aggressive act prompted Britain and France to declare war on Germany on September 3, 1939, marking the official start of the conflict. The invasion was part of Adolf Hitler's expansionist policy and violated previous agreements, leading to a rapid escalation of hostilities across Europe.