The two major powers were the Axis (Germany, Italy and Japan) and the Allies (the U.S., Britain, France, China and the USSR).
The war lasted six years, from 1939 to 1945.
Franklin D. Roosevelt was president when Pearl Harbor was attacked. Harry Truman, then the vice president, became the 33rd U.S. president when FDR died in 1945.
The numbers are staggering, with an estimated 45 million civilian and 15 million battle-related deaths.
Instability in Europe after World War I is widely believed to have helped create a climate where Hitler's National Socialist Party could rise to power.
In September 1939 Hitler invaded Poland, an act that resulted in both France and Great Britain declaring war on Germany.
In his 1925 memoir "Mein Kampf," Hitler predicted the war in Europe and Jewish persecution in Germany.
V-E Day came first on May 8, 1945. V-J Day followed on Sept. 2, 1945, the day Japan surrendered to the Allies.
The U.S. dropped nuclear bombs on Hiroshima and Nagasaki in August 1945. The Hiroshima bombing was the first ever deployment of an atomic bomb on populated areas.
Pearl Harbor was bombed on Dec. 7, 1941. The attack took approximately two hours and resulted in the deaths of more than 2,000 American service members.
By the end of the war, more than 30 countries across the globe were involved in the fight between the Allies and the Axis.
The D-Day invasion took place on the coast of Normandy, France. Through misdirection, the Germans believed the attack would occur in the Pas-de-Calais along the narrowest part of the English Channel between England and France.
Worldwide battle casualties are estimated at 25 million.
D-Day, the day more than 160,000 Allied troops invaded Western Europe, occurred on June 6, 1944.
It took one day for Congress to approve President Roosevelt's request. There was a single dissenting vote.
In 1935, Robert Watson-Watt was working on a means of destroying enemy aircraft. His efforts resulted in the development of radar technology.
The Air Force was recognized as its own military branch in 1947, years after the war ended.
Japan and Russia (formerly known as the Soviet Union) never signed a formal peace treaty, but they are still working on it. One bone of contention is a dispute over the South Kuril Islands (also called the Northern Territories dispute), which the Russians took from the Japanese after the war.
About 400 Navajo "code talkers" worked for the war effort. Their coded communications were never broken by the Japanese.
That would be Nutella, a chocolate and hazelnut spread that is experiencing a resurgence of popularity.
The quote often used to describe World War II is attributed to President Woodrow Wilson, but he wasn't actually talking about World War II. He was referring to World War I.
Six battleships were eventually restored. The California and the West Virginia were sunk and later raised and repaired. Two, the Arizona and the Oklahoma, never rejoined the fleet.
Jerry, or Jerries (plural), was slang for a German soldier during World War I that became more commonly used in World War II.
Life in the U.S. changed drastically after Pearl Harbor. Raw materials like rubber, steel and nylon were used in the war effort. From early 1942 to late 1945, no domestic automobiles, auto parts or trucks were produced.
Had it been necessary, the third bomb was scheduled for release over Tokyo.
The "Liberty Steak" was a common hamburger. Fearing the name was somehow pro-Nazi, clever marketers changed it for the duration of the war.
Hitler's nephew, William Hitler, served in the U.S. Navy during the war. He changed his name to William "Willy" Patrick Stuart-Houston.
Bockscar was named for its usual pilot Frederick Bock. On the actual day of the bombing, though, Bock was in another B-29 designed to take scientific measurements of the blast.
Approximately 2,800 Japanese pilots died in kamikaze raids during the war. Their efforts killed over 4,900 sailors and destroyed 34 U.S. ships.
The United Nations formed in 1945 to replace the floundering League of Nations. Only sovereign states can become UN members. This influential organization currently has 193 member states.