Answer:
Germany invading Poland
Step-by-step explanation:
World War II that occurred between 1939 to 1945, started when on September 1st, 1939, Hitler's Germany invaded. This resulted in Great Britain and France to go into the war against Germany, two days after setting up the beginning of World War II.
On September 17th the Soviet Union had joined the war by invading Poland, then other nations were joining subsequently on the principle of alliance and interest