Around 2,000 years before WC1, because of troubles in fighting the native trolls of the region, the High Elves appealed to the humans for aid and taught them magic. Many of the first men who were taught the arcane arts later founded the mystical city-state of Dalaran where they could practice their art in peace, and the magical prowess of their descendents soon rivalled that of the Elves.
During the events of WarCraft II, the High Elves were on the side of the Alliance against the Orcs; their disenfranchised rivals, the Trolls, naturally sided with the Horde, bent upon the conquest of Azeroth as pawns of the Burning Legion. The Elven Ranger Corps, fleets, magi, and weaponscrafting skills greatly aided the Alliance cause. Though the Horde was eventually defeated, the Elves suffered grievously. The ancient Runestone at the mystic island sanctuary of Caer Darrow was siezed by the Orcs and hewn into altars charged with demonic energy, in order to give magic tp the Orcs' dim-witted allies, the Ogres. The Horde later assaulted Quel'Thalas directly and burned many acres of the enchanted sylvan kingdom. Soon after the Horde's defeat during the events of Beyond the Dark Portal, the High Elves split from the Alliance, despite the sacrifice of many humans and dwarves to save much of the nucleus of the kingdom. The High Elves called their forces home and sealed off Quel'Thalas from the world with the enchanted Elfgates. However, many of their bretheren did not heed the call home and continued to serve the Alliance, living in Lordaeron or even the distant human kingdom of Stormwind.
During the events of WC3, the Undead Scourge, led by the damned Crown Prince Arthas Menethil of Lordaeron, came to Quel'Thalas with the intention of reviving the necromancer Kel'Thuzad, the founder of the Cult of the Damned and one of the Lich King's agents, in the mystical waters of the Sunwell. The High Elves, led by the Ranger General Sylvanas Windrunner, tried to stop the invasion best as possible, but with no avail. Silvermoon was captured, Kel'Thuzad revived into a fearsome Lich, and Windrunner herself transformed by Arthas into a banshee, another pawn of the Scourge and a slave to the Lich King's will — or so she was meant to be.
With the loss of Quel'Thalas, the elves of Lordaeron split into three new racial groups:
Many of the surviving High Elves, led by Prince Kael'thas Sunstrider, burned the woods of Quel'Thalas to give the Scourge as Pyhrric a victory as possible. With the Sunwell defiled, they embraced demonic magic to sate their addiction and became the Blood Elves, temporarily rejoining the Alliance out of necessity and in the hopes of enacting vengeance on the Scourge.
Along with Sylvanas Windrunner, many elves existed in undeath as the wrathful banshees, though some, like Sylvanas, were given back their corporeal forms. The zombified but sentient Sylvanas later rebelled from the Scourge during the events of the Frozen Throne and founded a faction of freethinking undead in and around the ruined kingdom of Lordaeron, calling themselves the Forsaken.
The high elves who were not in Lordaeron at the time of the kingdom's fall — those who had fled with Jaina Proudmoore to Kalimdor, or were elsewhere in the Eastern Kingdoms — rejoined the Alliance, as did some survivors of the undead invasion who did not approve of the Blood Elves.