The Lore of Warcraft

Teleros helpfully summarizes it from the very beginning:

1. Warcraft 1 sees the orcs ruin their homeworld through being blind to demonic interference, so they have to flee. Only by "flee" I mean "invade and conquer an innocent nation".
2. Warcraft 2 sees the orcs, now lead by someone who has an inkling of the demonic interference, try and murder their way across the other innocent nations, who are forced to band together as the Alliance to survive. The orcs meanwhile recruit money-grubbing (((goblin))) mercenaries and cannibal trolls who are pissed the Alliance races destroyed their empire & drove them out years before. The Alliance narrowly wins & proceeds to cut off the orc homeworld to stop reinforcements + demonic magic, and very emphatically do not genocide the orcs, instead putting them in camps at heavy (ie, unrest-causing) taxpayer expense.
3. Warcraft 3 sees the orcs try to flee to a new homeland. Under human-trained Thrall, they keep their violence to a minimum vs the Alliance, and ally together to defeat the demons. When an Alliance nation decides to try and solo the new orc homeland, the Alliance as a whole lets the Horde destroy their invasion force & kill their king in order to keep the fragile peace.
4. WoW basically starts with a cold war in place, with minor groups from both sides squabbling over bits and pieces (see: Battlegrounds), kind of like USA/Soviet proxy wars. Whilst Sylvanas prepares for genocide.
5. More cold war in The Burning Crusade.
6. War breaks out in Wrath of the Lich King when rogue Forsaken gas the combined Alliance/Horde army. Alliance capture Undercity, but Jaina tries for peace so teleports them out. A Horde army ambushes an Alliance army fighting the undead, resulting in an undead victory.
7. The Cataclysm happens. The war heats up. Alliance forces leave an opening for Tauren civilians to escape a besieged town, whilst Garrosh prefers killing civilians. Sylvanas gases more civilians and invades a neutral nation for ~reasons~.
8. Pandas. Alliance put neutral civilians to work to establish a base, Horde prefer to capture their children and such. Garrosh continues acting like Hitler, but is defeated and put on trial by both sides for war crimes. A treaty is made which should put an end to most of the territorial disputes.
9. Warlords of Draenor, in which we learn that the orcs were never a peaceful race before the demons came but in fact the Mongols on 'roids, and easily persuaded to go invade Azeroth for the lulz.
10. Legion, in which the 2nd nice Horde leader dies and Sylvanas takes over. Alliance forces disobey orders to attack her, but all they really do is foil her plan to enslave the valkyries.
11. Battle for Azeroth, in which Sylvanas decides on pre-emptive war to control the world's supply of plutonium - I mean Azerite - and starts this by firebombing Teldrassil without even the pretence of asking for a surrender. Later irradiates / gases her own troops to deny the Alliance control over her undead capital.