People maintain the common mistakes of calling High Elves and Humans ‘centuries-long allies’
What their relationship was, was nothing more but: “You can be useful… we’ll remember that you exist when we need you.”
The High Elves sought the humans out out of desperation when the Amani launched their invasions plans, not before, and not because of any love for the humans. They struck a deal with the humans: “Help us, and we’ll teach some of your people magic” the dislike for the humans further showed itself, when the High Elves quickly seized the opportunity to again take the magic they taught the humans away from them (Well, attempted to), following the human’s misuse nearly bringing back the Legion.
There was no deeper friendship between the two, the humans may have wanted more, but the larger population of High Elves never did. They even mistrusted the humans so much, that they even looked at their own crown prince with skepticism due to his talk of cooperation with the Alliance - even Anasterian Sunstrider feared for Kael’thas’ loyalties.
All this bullcrap about how the High Elves were all ‘buddy-buddy’ with the Alliance is exactly that, bullcrap.
The Blood Elves, at the end of the day, should not have been allied with either faction. The High Elves, should not be allied with either faction. When Kael’thas was first forced into a desperate situation, and then to be executed in the city of Dalaran, the Blood Elves and High Elves should have abandoned the Alliance then and there.
Under the eyes of two Dalaran leaders, Modera and Ansirem, members of the Kirin’tor council at the time, Kael’thas was to be executed. And under the eyes of the Ironforge dwarves too.
Anywho, the story that is told thus far, is fairly sensible. If anything, the Blood Elves and High Elves should in fact align themselves more so with the Horde than the Alliance. At the time of the Burning Crusade, the forsaken was lead by Sylvanas Windrunner, who at the time could be seen as a fair leader, and who had direct connections with High Elves and Blood Elves both. She was the ranger-general who gave her life in an attempt to save the High Elven people, now leading undead who broke away from the Lich King’s, Arthas’ control.
The Horde was newly-founded, the Blood Elves very much aware of the demon blood and legion control that first forced the orcs to the borders of Quel’thalas. And with the Ironforge dwarves being members of the Alliance, the same faction of dwarves that gladly saw their prince and kin executed, there is no way that they could trust the Alliance wholeheartedly. The Alliance’s shenanigans in Eversong Woods and the Ghostlands was unnecessary, but served to be the final drop.
With the Blood Elves desperately needing aid against the remnants of the scourge and amani uprising, the Alliance came to trample on them, the Horde came to help. The Blood Elves also needed help reaching Outland, and the Horde needed the Blood Elves for information on stuff in Outland.
The Void Elves is a story gone wrong. While I find their existence to be fine, as it is a great compromise to High Elves. The Void Elves should have been the Silver Covenant, and the Allerian Outpost High Elves being inspired by news of Alleria Windrunner - and thus they pursued her and similar powers - only to end up in a trap (As the story goes). Making it a ragtag of Blood Elves was a mistake, and served only to say: “Look guys, the Blood Elves are totally Horde! No need to doubt it, the last remnants of Alliance sympathy is gone now” that was just stupid - but the entirety of BFA seems to have ignored Lor’themar’s character anyway, he and Baine agreeing to continue a senseless war was just stupid.
But that is the factions wars in a nutshell - for them to work other members of either faction would have to be written to be dumber than they actually are. The Orcs, Tauren, Trolls, Blood Elves and Nightborne could have pulled out of the BFA war at any moment, leaving Sylvanas with a small faction of Goblins and Undead. The five races that left could form their own coalition in defense against Sylvanas’ retaliation, basically forcing her into a prolonged fight on two fronts which would not go well for her at all, against forces significantly larger than her own no less.
Damnit, I hate this story.