Final answer:
The German invasion of Poland on September 1, 1939, unleashed World War II, with Britain and France declaring war on Germany as a result.
Step-by-step explanation:
The event that finally unleashed World War II was the German invasion of Poland on September 1, 1939. This invasion was a breach of international law and numerous treaties. Despite attempts at appeasement, such as the Munich Agreement in 1938, German aggression continued unabated. The Polish campaign showcased the devastating speed and effectiveness of the German military tactic known as Blitzkrieg, resulting in a swift defeat of Polish forces.
Before this direct conflict, Hitler's rise to power and consequential territorial expansions, such as the annexation of Austria and the occupation of the Sudetenland, significantly increased tensions in Europe. Additionally, the Japanese invasion of China in 1937 escalated conflict in Asia, which is considered by some historians as the beginning of World War II in the Eastern theater. However, it was the attack on Poland that precipitated a global conflict with Britain and France declaring war on Germany.