World War II began on September 1, 1939, when Germany invaded Poland. This aggressive act by Adolf Hitler's regime marked the start of a global conflict that would encompass many nations and lead to significant shifts in power and territory. The invasion prompted Britain and France to declare war on Germany, escalating into a larger war that involved numerous conflicts across Europe, Asia, and beyond. The war would ultimately lead to profound changes in geopolitical dynamics, the rise of the United States and the Soviet Union as superpowers, and extensive international cooperation through organizations such as the United Nations.