The Sin'dorei have been a presence in WoW since The Burning Crusade -- and surprisingly enough, as members of the Horde rather than members of the Alliance. This turnaround in events was largely due to the treatment of the blood elves by the Alliance during Warcraft III. Kael'thas Sunstrider watched as his people were slaughtered by the Scourge, and set out to lend a helping hand to his supposed allies, hoping that they would lend a hand in return. However, he was sent to help Garithos, a man who was -- let's face it -- incredibly racist.
And in the face of that not-quite-blatant racism, Kael'thas turned to the only people offering any sort of real alliance; the naga. While Vashj and company helped Kael'thas far more than any of his supposed Alliance allies, Garithos was happy to find an excuse to condemn the leader of the sin'dorei, and had him imprisoned in Dalaran for his supposed treasonous actions. It was this waterfall effect that eventually led to the sin'dorei's withdrawal from the Alliance, and into the arms of the Horde.
Which makes the events of patch 5.1 all the more ironically interesting ... because it's happening all over again, but wearing a slightly different face.
Please note: This Know Your Lore contains spoilers for Tides of War as well as patch 5.1 content from both Operation: Shieldwall and the Dominance Offensive. If you have yet to complete these stories, you may want to veer away.