The significant global conflict that began in 1939 was World War II. Sparked by Germany's invasion of Poland on September 1, 1939, it involved numerous countries and resulted in vast casualties and destruction. The war was characterized by the opposing alliances of the Allies, including the United States, the United Kingdom, and the Soviet Union, against the Axis powers, primarily Germany, Italy, and Japan. World War II fundamentally reshaped the geopolitical landscape, leading to the emergence of the U.S. and the Soviet Union as superpowers and significant changes in international relations, eventually paving the way for the Cold War.