World War II began when Germany invaded Poland in 1939, which caused France and Great Britain to declare war on Germany. Germany conquered a significant part of Europe from that time until 1941, but in 1942, the Allies halted the advance. Each member of the Axis powers surrendered in 1945.
Although World War II began in 1939, conflicts related to it occurred as early as 1931, when Japan invaded Manchuria. Germany, Italy and Japan were the primary members of the Axis powers who fought against the Allies in the war.
France and Great Britain guaranteed Poland's borders, leading to the war in Europe after Germany's invasion. France was conquered along with several other European nations in 1940, but Great Britain defeated Germany in the Battle of Britain and wasn't conquered. Despite signing a nonaggression agreement with the Soviet Union in 1939, Germany invaded it in 1941. Russia's size and cold weather conditions during the winter lead to Germany's retreat in 1943.
Although Japan was already at war with China, it attacked Pearl Harbor in 1941, drawing the United States into the war. The British and U.S. navys stopped Japan's advance in the Pacific in 1942.
Italy surrendered in 1943, after removing Mussolini from his position as prime minister. The Allies invaded France on June 6, 1944. Due to the Allies' victories, Germany surrendered on May 7, 1945. The war ended when Japan formally surrendered on Sept. 2, 1945, after the United States used atomic bombs on Hiroshima and Nagasaki.