Which event caused the outbreak of ww2 in September 1939?

In the late 1930s Poland found itself between two expansionist powers, Germany and the Soviet Union. Germany wanted to invade Poland but feared that such an invasion would spark a war with the Soviet Union. This fear was removed when Germany and the Soviet Union signed the Molotov-Ribbentrop Pact on August 23, 1939. One of the secret clauses in the pact was the provision that Germany and the Soviet Union wound mutually invade Poland and carve up its territory between them.

Fearing for its safety, Poland entered into the Polish-British Common Defense Pact on August 25, 1939. The pact declared that the UK would defend Poland if invaded by Germany. A similar agreement was already in place between Poland and France. On the night of August 31, 1939 German operatives, dressed as Polish soldiers, created a false flag by seizing a German radio station at Gleiwitz in the German border province of Upper Silesia. The Germans used this "attack" as a pretext to invade Poland. The following day, September 1 1939, Germany troops crossed the Polish frontier. On September 3, 1939, Britain and France declared war on Germany.