The title says it all.
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EDIT: Story of Warcraft in one minute.
Prologue: Sargeras betrays the titans, builds a demonic army, takes over Argus (Velen and his followers escape), attempts to invade Azeroth but Broxigar stops him. Kalimdor is split into fragments, creating the continents of Azeroth.
WC1: Medivh betrays Azeroth (He's demon possessed), summons a portal for a Kil'jaeden motivated Orc army to invade. Eastern Kingdoms unite into an Alliance and repel the invasion.
WC2: Renewed Orcish effort to fight the Alliance. Alleria and Turyalon sacrifice themselves and their armies on a one-way-trip to deactivate the Dark Portal. Draenor is nuked by ridiculously extreme magical focus. Orcs enslaved.
WC3: Thrall leads Orcs to Kalimdor, founds the New Horde. Grommosh sacrifices himself to save the Orcs. Arthas gives into the Legion and rampages across the Eastern Kingdoms before taking over Northrend. Kael'Thlas flees to Outland. Archimonde leads a second invasion into Azeroth but is slain.
WoW: Horde and Alliance go to war against each other. War of the Sands takes place.
BC: Alliance and Horde invade Outland. Kael'thas summons Kil'jaeden to Azeroth, but both are defeated. Garrosh is convinced of his father's wish and moves to Azeroth.
WotLK: Tirion's crusade against the Scourge. Putress betrays Azeroth and assists Varimathras's coup. Both are defeated. Ulduar discovered; Alagon returning to Azeroth regularly to observe. Arthas defeated. Bolvar becomes the new Lich King.
Cata: Carine assassinated by Magtha; Magni frozen. Renewed war between the Horde and the Alliance. The Cataclsym reshapes the world. Deathwing is slain. The aspects become mortal.
MoP: Theramore anhilated. Pandaria discovered. Garrosh unearthes the heart of an old god, and tries to purge the Horde. He is defeated and overthrown. Wrathion assists in his downfall and prepares Azeroth for an 'ultimate invasion' by the Legion.
WoD: Wrathion and Kairoz betray Azeroth and free Garrosh, sending him to the past. This creates an alternate Draenor where the world was not nuked and the Orcs were not enslaved to Kil'jaeden. Iron Horde invades Azeroth, but is pushed back. Expedition into Draenor to destroy the Dark Portal is launched.