What significant conflict began in 1939 and involved major global powers?

The significant conflict that began in 1939 and involved major global powers is World War II. This global war saw the extensive involvement of many countries, with key players including the Allies and the Axis powers. It started with the invasion of Poland by Germany on September 1, 1939, which prompted Britain and France to declare war against Germany shortly thereafter.

This conflict is marked by its scale, with battles occurring across Europe, Asia, Africa, and the Pacific, significantly altering political, social, and economic landscapes around the world. The outcomes of World War II led to the establishment of the United Nations and set the stage for subsequent global conflicts, including the Cold War. Understanding the timeline and impact of World War II is crucial for historical context, as it was a pivotal moment in world history that shaped the modern era.