Final answer:
World War II began with the German invasion of Poland in September of 1939. Varying perspectives exist, with some historians marking the start at the Japanese invasion of China in 1937. The war involved global powers and resulted in significant loss of life and geopolitical changes.
Step-by-step explanation:
The onset of World War II is commonly marked by Germany's invasion of Poland, which occurred in September of 1939. This military action by Germany was in direct defiance of the Treaty of Versailles' provisions, which aimed to keep peace following World War I. Adolf Hitler's regime had been preparing for war, growing its military might, before this event. The attack on Poland led Britain and France to declare war against Germany, signaling the start of the war across Western Europe. However, perspectives on the start of the conflict vary globally, with Asian historians citing the Japanese invasion of China in 1937 as the beginning, emphasizing a broader understanding of the war's scope.
Despite varied viewpoints on when the war began, it is widely accepted that World War II was a major global conflict involving numerous countries and resulting in dramatic changes to the geopolitical landscape. The Allies, comprised of the United States, the Soviet Union, Britain, and others, eventually defeated the Axis powers, which included Germany, Italy, Japan, and their allies. The war had devastating effects, with estimates of around eighty million fatalities, and saw significant advancements in weaponry and military tactics alongside atrocities such as the Holocaust.