Quiz Answers – World War II Question #21: What event in 1939 is considered the opening of the war because it caused most of Europe to declare war on Germany? Correct Answer: The German invasion of Poland Pages: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37