Seven decades after Nazi Germany's attack on Poland, the answer depends on where you are standing.
Most historians consider that the war broke out on September 1, 1939, when a German battleship opened fire on a Polish fort in Danzig (modern-day Gdansk), and Nazi forces swept across Poland's borders, without a formal declaration of war.
But it remained a German-Polish conflict until September 3 when Britain and France, bound to Poland by military pacts, declared war on Germany, pulling their vast empires into the war.
World War II was eventually formally to involve almost every nation of the planet, to varying degrees. An alternative viewpoint comes from Asia.
China's then leader, Chiang Kai-shek, in his diaries traced the start of World War II to 1931, when Japan seized Manchuria in northeast China and turned it into a puppet state.
Others in China consider the war began on July 7, 1937, when Japanese forces launched their all-out assault on the rest of China.
In Russia, meanwhile, what is known as the Great Patriotic War started on June 22, 1941, when Nazi troops invaded the Soviet Union.
Poles, however, remember that Soviet troops entered the east of their country on September 17, 1939, under the terms of the Molotov-Ribbentrop Pact with the Nazis.
In addition, Soviet troops had fought the Japanese in 1938 and 1939, attacked Finland weeks after moving against Poland, and also took over the Baltic states of Estonia, Latvia and Lithuania in 1940.
For West Europeans, the conflict remained the "Phoney War" until 1940, when the Nazis swept through Denmark, Norway, Belgium and Holland, and attacked Britain from the air.
Germany's ally, Fascist Italy, also entered the conflict in 1940.
For the US, meanwhile, the war began on December 7, 1941, with the Japanese bombing of the US Navy's base at Pearl Harbour in Hawaii.
There are also differences over the war's end.
In Western Europe, the focus is on May 8, 1945, while in Russia the date is May 9, when Germany signed its surrender.
The Pacific war, meanwhile, ended on September 2, when Japan surrendered to the United States after the atomic bombing of Hiroshima and Nagasaki.