Just two decades after the end of World War I, World War II began in the summer of 1939 with Germany’s invasion of Poland the morning of September 1. According to Encyclopedia Britannica, World War II was a continuation of the unsettled conflicts that began in World War I.
The German-Soviet Nonagression Pact was signed on August 23, 1939. Once Adolf Hitler had this assurance of success for Germany's intended takeover of Poland, he was eager to invade, and on August 31, he gave orders for hostilities to begin the next morning. His orders were carried out as planned, and two days later, Great Britain and France declared war on Germany in response.