The major global event that took place in 1939, leading to widespread conflict in Europe, was the outbreak of World War II. The war began on September 1, 1939, when Germany, led by Adolf Hitler, invaded Poland, prompting Britain and France to declare war on Germany shortly afterward. This aggression marked the start of a six-year conflict that involved numerous countries and led to significant changes in political boundaries, war atrocities, and the eventual establishment of the United Nations. World War II reshaped the global landscape, leading to the rise and fall of empires and initiating the Cold War era.