According to the protagonist in Kurt Vonnegut's "Timequake":

"...The world's second unsucessful attempt to commit suicide."

Germany invades Czechoslovakia.
Britain & France tell them to stop that bullshit.
Germany invades Poland.
(Russia also invades Poland from the other side: everybody forgets this.)
Britain & France declare war. This is the 'official' kick-off.
Italy, Bulgaria, Hungary, & Romania all join the German side. (Everybody forgets the last three.)
Axis forces go through Europe like vindaloo through a colostomy.
Nazis exterminate Jews, gays, gypsies, & the disabled. (everybody remembers the jews but forgets the rest.)
UK holds out.
Russia & the USA don't do shit.
Entire divisions of Danish, Belgian, Dutch, Norwegian, French & Serbian volunteers join the Axis armies & SS. (everybody forgets this & to listen to them now, they were all in the fucking resistance, which must have been MASSIVE.)
Axis forces invade Russia. Suddenly the Russians don't think it's funny any more.
Japan joins the Axis & bombs Pearl Harbor.
Suddenly the US doesn't think it's funny any more.
The USA tools up the world, 'cause it's got more factories than everybody else put together, & they're out of bomber range.
Axis runs out of steam in Russia, cause Russia's enormous & bloody freezing.
Allies invade on D-Day... 5 landings: 2 British, 2 American, 1 Canadian. (everybody forgets the Canadians.)
Hitler ends up smouldering in a ditch. Russians find the body & confirm he only had one ball. Seriously.
The US decides invading stuff is a pain in the ass and invents the atom bomb instead. Drops two ...

One of the few sequels that surpassed the original.

Allies vs Axis, started mostly by fascist imperialism on the part of the Axis powers. During the war, Germany at its peak had taken over most of Europe, Japan had taken most of the Pacific island nations, and Italy had taken most of northern Africa. Ended in 1945. Perhaps the most significant event of the 20th century, and this dictionary can't possibly begin to cover all of it. See also: every show on the History Channel.

Briefly: A war in which aproximately 12 million allied soldiers and 33 million civilians lost their lives defending the freedoms many now take for granted.

A war in which over 56 million lives were snubbed out and people like to debate about who made the biggest contribution in winning against the axis powers.
1939 - 1945 NOT 1937.

Country Men in war Battle deaths Wounded
Australia 1,000,000 26,976 180,864
Austria 800,000 280,000 350,117
Belgium 625,000 8,460 55,5131
Brazil2 40,334 943 4,222
Bulgaria 339,760 6,671 21,878
Canada 1,086,3437 42,0427 53,145
China3 17,250,521 1,324,516 1,762,006
Czechoslovakia — 6,6834 8,017
Denmark — 4,339 —
Finland 500,000 79,047 50,000
France — 201,568 400,000
Germany 20,000,000 3,250,0004 7,250,000
Greece — 17,024 47,290
Hungary — 147,435 89,313
India 2,393,891 32,121 64,354
Italy 3,100,000 149,4964 66,716
Japan 9,700,000 1,270,000 140,000
Netherlands 280,000 6,500 2,860
New Zealand 194,000 11,6254 17,000
Norway 75,000 2,000 —
Poland — 664,000 530,000
Romania 650,0005 350,0006 —
South Africa 410,056 2,473 —
U.S.S.R. — 6,115,0004 14,012,000
United Kingdom 5,896,000 357,1164 369,267
United States 16,112,566 291,557 670,846
Yugoslavia 3,741,000 305,000 425,000

All of the above except for the axis powers contributed to the end of the war and none should have there bravery taken away by overpatriotic fools.
To all our glorious dead. Lest we forget

This covers the lead up to war and the war:

Japan invades Manchuria and aims to take the rest of China under the first two stages of the Tanaka Memorial (this starts the Second Sino-Japanese War), Hitler becomes Chancellor, Hinderburg dies-Hitler becomes Fuhrer, readies to invade Austria, but Mussolini mobilises and readies to go to war with Hitler if he invades (not many people know this), Anglo-German Naval Agreement, Britain allows hitler to build the Kriegsmarine to 35% the total tonnage of the Royal Navy, Mussolini invades Ethiopia, and hitler invades the Rhineland at the aame time, Spanish Civil War, Condor Legion of Germany and the Aviazione Legionaria of Italy help the Spanish Nationalists (in fact Germany and Italy were testing their militaries), Italy, Japan and German sigh a few treaties creating the Axis powers, Mussolini lets Hitler take Austria this time (the Austrian people wanted Anschluss, but the voters are kept under close observation by German Soldiers), Czechoslavkia, Britain and France tell Hitler to stop it, Hitler agrees but invades the rest six months later, Hitler invades Poland, angering Britain (Britain and Poland had a treaty), Britain gives Hitler and ultimatum, Hitler ignores it, Britain goes to war, France goes to war, Britain bombs German warships on the Kiel canal (not many people know this), Soviets invade Eastern Poland and sign the Russo-German Non-Agression Pact, Russian invades Finland for no reason (The Winter War), Russia condemned...