Final answer:
The period during October 1939-April 1940 for the Germans was known as the "Sitzkrieg" or the "phony war," characterized by a lack of direct conflict on the Western Front after the declaration of war by France and Britain but before the surprise invasion of Denmark and Norway.
Step-by-step explanation:
The "Phony War"
The period during October 1939-April 1940 for the Germans was called the "Sitzkrieg", also known as the "phony war". Following the invasion of Poland, France and Britain had declared war on Germany, setting the stage for World War II. However, during this time, there was a lack of direct conflict on the Western Front, leading to the sense that although war had been declared, it was not actually being waged. This deceptive calm ended when Germany launched a surprise invasion of Denmark and Norway in April 1940, marking the end of the "phony war" and the escalation into full-scale warfare.