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World War II started in 1939 when Germany invaded Poland, leading to a global conflict.

Key events included the Battle of Britain, Pearl Harbor attack, and D-Day invasion.

The war ended in 1945 after Germany and Japan surrendered following significant battles.

World War II (WWII) was a long and bloody war that lasted about six years. Officially beginning on September 1, 1939, when Germany invaded Poland, World War II lasted until both the Germans and the Japanese surrendered to the Allies in 1945.

The war encompassed major global conflicts and pivotal moments such as the Holocaust and the use of atomic bombs on Hiroshima and Nagasaki, and it was marked by significant events including the Battle of Britain, the attack on Pearl Harbor, and the D-Day invasion.

Here is a timeline of major events and key dates during the war.

1939

September 1 may be the official start of World War II, but it didn't start in a vacuum. Europe and Asia had been tense for years before 1939 because of the rise of Adolf Hitler and the Third Reich in Germany, the Spanish Civil War, the Japanese invasion of China, the German annexation of Austria, and the imprisonment of thousands of Jews in concentration camps. After Germany's occupation of areas of Czechoslovakia not previously agreed to in the Munich Pact and its invasion of Poland, the rest of Europe realized it couldn't try to appease Germany any longer. The United States tried to remain neutral, and the Soviet Union invaded Finland.

London after an air raid during the London Blitz, 15th October 1940. Central Press/Getty Images​ 1940

The first full year of the war saw Germany invading its European neighbors: Belgium, the Netherlands, France, Denmark, Norway, Luxembourg, and Romania. Further, the bombing of Britain lasted for months. The Royal Air Force undertook nighttime raids in Germany in response. Germany, Italy, and Japan signed a joint military and economic agreement, and Italy invaded Egypt, which was controlled by the British, Albania, and Greece. The United States shifted to a stance of "nonbelligerancy" rather than neutrality so it could find ways to help the Allies, and the Lend-Lease Act (the exchange of materiel aid for strategic leases on foreign military bases) was proposed late in the year. Popular opinion still didn't want Americans in another war "over there." The Soviet Union, meanwhile, took part of Romania and installed Communists in the Baltic States, later annexing them.

German soldiers with Russian prisoners, Russia, 1941.  Print Collector/Getty Images 1941

The year 1941 was one of escalation around the world. Italy may have been defeated in Greece, but that didn't mean that Germany wouldn't take the country. Then it was on to Yugoslavia and Russia. Germany broke its pact with the Soviet Union and invaded there, but the winter and Soviet counterattack killed many German troops. The Soviets next joined the Allies. Within a week of the Pearl Harbor attack, Japan had invaded Burma, Hong Kong (then under British control), and the Philippines, and the United States was officially in the conflict.

Aircraft Carrier Yorktown Being Hit by Japanese Bomber during Battle of Midway. Bettman/Getty Images  1942

U.S. troops first arrived in Britain in January 1942. Also that year, Japan captured Singapore, which was Britain's last location in the Pacific, as well as islands such as Borneo and Sumatra. By the middle of the year, though, the Allies started gaining ground, with the Battle of Midway being the turning point there. Germany captured Libya, but the Allies started making gains in Africa, and Soviet counterattacks made progress as well in Stalingrad.

German POWs in Stalingrad in 1943. Historical/Getty Images  1943

Stalingrad turned into Germany's first major defeat in 1943, and the North Africa stalemate ended, with the surrender of the Axis powers to the Allies in Tunisia. The tide was finally turning, though not fast enough for the people in the 27 merchant vessels sunk by Germany in the Atlantic in four days in March. Nevertheless, Bletchley codebreakers and long-range aircraft inflicted a serious toll on the U-boats, pretty much ending the Battle of the Atlantic.

In the fall, Italy fell to Allied forces, prompting Germany to invade there. The Germans successfully rescued Mussolini, and battles in Italy between forces in the north and south drug on. In the Pacific, Allied forces gained territory in New Guinea—to attempt to protect Australia from Japanese invasion—as well as Guadalcanal. The Soviets continued expelling Germans from their territory, and the Battle of Kursk was key. The end of the year saw Winston Churchill and Josef Stalin meeting in Iran to discuss the invasion of France.

1944

American troops played a big role in battles to take back France in 1944, including landings on Normandy beaches that caught the Germans by surprise. Italy was finally liberated as well, and the Soviets' counterattack pushed the German soldiers back to Warsaw, Poland. Germany lost 100,000 soldiers (captured) during the battle in Minsk. The Battle of the Bulge, however, postponed the Allies marching into Germany for a while.

In the Pacific, Japan gained more territory in China, but its success was limited by the Communist troops there. The Allies fought back by taking Saipan and invading the Philippines.

ors Survivors of Auschwitz leaving the camp at the end of World War II, Poland, February 1945. Galerie Bilderwelt/Getty Images 1945

The liberation of concentration camps, such as Auschwitz, made the extent of the Holocaust clearer to the Allies. Bombs still fell on London and Germany in 1945, but before April was over, two of the Axis leaders would be dead and Germany's surrender would soon follow. Franklin D. Roosevelt also died in April but of natural causes.

The war in the Pacific continued, but the Allies made significant progress there through battles at Iwo Jima, the Philippines, and Okinawa, and Japan started to retreat from China. By mid-August, it was all over. Japan surrendered shortly after the second atomic bomb was unleashed on the island nation and on September 2, the surrender was formally signed and accepted, officially ending the conflict. Estimates put the death toll at 62 and 78 million, including 24 million from the Soviet Union, and 6 million Jews—60% of all the Jewish population in Europe.