World War II, which began with Germany's invasion of Poland in 1939 and evolved into a global conflict, occurred between two major military coalitions known as the Allies and the Axis powers. The expansionist policies of Adolf Hitler, the leader of Nazi Germany, were among the main causes of the war. To understand the main stages of World War II, it is essential to focus on key events.

The invasion of Poland by Germany in 1939 marked the outbreak of the war. The rapid occupation of Poland using the Blitzkrieg tactic led to France and the United Kingdom declaring war on Germany. The Blitzkrieg tactic on the Western Front resulted in the occupation of the Benelux countries and France by Germany. Despite the Dunkirk Evacuation allowing partial withdrawal of British and Allied forces, Germany gained control in Western Europe.

In 1941, Germany launched Operation Barbarossa, invading the Soviet Union. While German forces initially achieved success, harsh winter conditions and Soviet resistance halted their advance. In the same year, Japan's attack on Pearl Harbor prompted the United States to enter the war. Conflicts occurred in the Pacific Theater between the U.S. and its allies against Japan.

From 1942 to 1943, Allied forces engaged in battles in North Africa and the Italian Front against German and Italian forces. The invasion of Italy and the overthrow of Mussolini led to Italy joining the Allies. In 1944, the Normandy Invasion marked the Allied counterattack in Western Europe.

In the Eastern Front from 1944 to 1945, Soviet forces advanced towards Germany, eventually besieging Berlin and contributing significantly to the end of the war. The dropping of atomic bombs on Hiroshima and Nagasaki by the United States expedited Japan's surrender.

The conclusion of World War II left the world in great devastation, claiming millions of lives and fundamentally altering the global political landscape. The establishment of the United Nations marked the post-war era, initiating the Cold War as the world became divided between two major blocs. World War II is remembered as one of the largest conflicts in human history.