Which country did Adolf Hitler invade in September 1939, marking the start of World War II in Europe?

The invasion of Poland by Adolf Hitler in September 1939 is widely recognized as the event that marked the beginning of World War II in Europe. This military action was a calculated move by Nazi Germany to expand its territory and assert dominance in Europe. The invasion was swift, employing the Blitzkrieg tactics that demonstrated the effectiveness of combined arms warfare, including rapid movement of infantry, tanks, and air support.

The response from the Allied powers was immediate; both the United Kingdom and France declared war on Germany within days of the invasion. This act of aggression set off a chain reaction of events that involved multiple nations and escalated into a full-scale global conflict.

The other countries mentioned, such as France and Belgium, were invaded later in the war, while Czechoslovakia had already been occupied through diplomatic pressure and military intimidation by Germany prior to the official start of the war. Thus, the invasion of Poland is historically significant as it officially initiated the hostilities that defined the Second World War in Europe.