Which of the following events of World History marked the beginning of the Second World War?

On 1 september 1939 German armies marched into poland. On 3 September Britain and France declared war on Germany.Thus the invasion of poland marked the beginning of the Second World War. The German armies completed the conquest of poland in less than three weeks as no aid reached poland. In spite of the declaration of war, however, there was little actual fighting for many months. Therefore, the war during the period from  september 1939 to April 1940 when germany invaded Narway and Denmark is known as the "Phoney War"