It was the world's deadliest conflict, estimated to have killed 40 to 60 million people, more than half of whom were civilians.
Here are key dates in the war.
Nazis invade Poland
Adolf Hitler's Nazi forces invade Poland from the west on September 1st, 1939, prompting Britain and France to declare war on Berlin two days later.
Soviet forces attack Poland from the east on September 17th, under a secret pact with Germany, and the city of Warsaw surrenders 10 days later.
Archive photo shows Adolf Hitler in Poland. Photo: DPA
Northern France occupied
After occupying Denmark and Norway, Hitler launches a major offensive against Belgium, The Netherlands, Luxembourg and France in May 1940.
France's wartime leader Philippe Petain on June 17th announces its capitulation; on June 22nd he signs the armistice that allows Nazi forces to occupy the north of the country.
On June 18th French General Charles de Gaulle calls from London for resistance.
Petain sets up a government headquartered in the central spa town of Vichy that collaborates with the occupying German forces.
Battle of Britain
On August 13th, 1940, the Battle of Britain begins with German bombers carrying out massive attacks on cities such as Coventry and London, the raids continuing for nine months.
Faced with British resistance, Hitler abandons his planned invasion of Britain and declares, later in August, a blockade of the British Isles with his submarine fleet.
Soviet Union attacked
On June 22nd, 1941, Berlin turns east and launches an attack against the Soviet Union.
The German army is halted at the gates of Moscow in December by a Soviet counteroffensive. However its siege of Leningrad, now Saint Petersburg, will last nearly 900 days until January 1944.
On December 7th, 1941, Japan — allied with Germany — attacks the US military base in Pearl Harbor, Hawaii, destroying most of the US Pacific fleet.
The Americans, led by Franklin D. Roosevelt, enter World War II the next day.
US ships sinking after severe damage during the attack on Pearl Harbor on December 7, 1941. Archive photo: DPA
On January 20th, 1942, Hitler ramps up at a Berlin conference his “Final Solution” plan to exterminate Jews, in particular by mass killings in concentration camps.
Some six million Jews are slain, as well as many political prisoners,
including resistance fighters, homosexuals and gypsies.
North Africa defeat
In October 1942 British troops defeat the German Afrika Korps in Egypt, handing the Nazi army its first major setback.
In November thousands of British and American forces land in North Africa,
leading to the surrender in 1943 of German-allied troops in the region.
Battle of Stalingrad
On February 2, 1943, the five-month Battle of Stalingrad ends with Soviet victory over the Nazis.
It is the first Nazi surrender in Europe since the war began, and costs the German army half a million men.
The battle for Stalingrad, now renamed Volgograd, was the first devastating defeat for the German Wehrmacht in the war against the Soviet Union in 1943 and became a turning point on the Eastern Front. Archive photo: DPA
D-Day in France
In November 1943 the Allies agree to attack Germany via occupied France.
D-Day is on June 6th, 1944, when more than 156,000 mainly American, British and Canadian troops land on the Normandy beaches. They overwhelm the Germans and liberate Paris on August 25th.
The Soviet army arrives in Berlin in April 1945 and captures the city by early May.
Hitler commits suicide on April 30th. On May 8th Germany surrenders unconditionally, ending the war in Europe.
On August 6th and 9th US aircraft drop atomic bombs on the Japanese cities of Hiroshima and Nagasaki respectively.
On September 2nd, 1945, Japan officially surrenders. World War II is over.