*facepalm* Illidan followed that Demon guy who was the new legion's commander. ITS FUCKING SAID IN A CINEMATIC!
You fail to understand me again and the story again. Garithos was the only one leading an opposition to the undead at the time. He needed those reinforcment for the front line because if they lose there than dalaran is lost. Kael however was sent to fight off a SCOUTING group,not a main army just some scouts. If he failed,he would have been avenged and dalaran would still be in allied hands.Its pritty obvious he wasnt controled because in that mission when you get to kill the dreadlord, he and all of his army fight ASIDE the undead and have a zombified like mind.Where as before they all crearly have thier minds. I belive elfs could be mind controlled to but even if they werent and Garithos wanted him dead from the start, he and the undead would have just killed em.
- Illidan accepted a job from Kil'jaden to destroy The Frozen Throne.
- To accomplish this he called upon the Naga, and went to Dalaran to use the Eye of Sargeras. In Dalaran he failed, so he retreated to Outland.
- We don't know whos idea was it, but Vashj convinced Kael to join Illidan.
- Illidan had no interest in the war between Undead and Humans.
- Illidan slayed Tichondrious in RoC.
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- We do not know clearly when, but at one point Garithos was sure to be mind controlled.
- If game statistics are any indication, mind control works only on the weaker minds. Garithos himself was already a racist and mostly a brute. So it was easier to control his mind and make him take certain decisions which wouldn't look strange.
- Kael had to defend Dalaran from a huge army. Knowing this, Garithos simply choose to take all the non-blood elven army units to the front, without stating exactly why. An elusive decision indeed.
- Varimathras was not the only Dreadlord. There were others as well. As the Burning Ligeon moved towards Kalimdore, they left 3 Dreadlords to keep an eye on the Undead in Lordaeron, and continue their campaign in the Eastern Kingdoms.
- The Burning Ligeon took a part of the Undead army with them to Kalimdor. Wether it was a huge chunk or just a small one, we cannot know. Though assuming that Archimonde himself was the leader of the army, and that their joint army of Demons and Undead wrecked havoc all the way to the World Tree through 3 races, even if it was just the survivors of Lordaeron, a really big part of the Horde was there, and most of the Night Elves. That leads to the conclusion, that the Ligeon took a big chunk of the Undead army with them.
- The Undead Scourge army being weakened in numbers, and facing an army formed of Dwarves from Aerie Peak and Khaz Modan, Blood Elves and Humans from Stormwind, they couldn't just roflstomp the mortals. Thus I believe, that the 3 Dreadlords decided to play it safe, and just defeat their enemies methodically, step by step. Their goal was to recapture Dalaran, so they mind control Garithos into leaving just the Blood Elves to defend the City while Garithos took all his men to the front lines.
- Eventually there weren't many human forces alive, so Detheroc could enslave them all.