The years 1939 to 1945 mark the period of World War II, a global conflict involving most of the world's nations. It began on September 1, 1939, when Germany, led by Adolf Hitler, invaded Poland. This prompted Britain and France to declare war on Germany, leading to widespread battles across Europe, Asia, and the Pacific. The war saw the rise of the Axis Powers, including Germany, Italy, and Japan, against the Allied Powers, which included the United States, Soviet Union, and United Kingdom. The conflict ended with the unconditional surrender of Germany in May 1945 and Japan in September 1945, resulting in significant geopolitical changes worldwide.