One can argue the Alliance burned mending bridges, although that brings in a whole other conversation that I believe has been had in this thread. Namely, Garithos was not a representative of the Alliance with any kind of authority to act on it’s behalf, only to lead it’s military efforts, as well as the fact the man was being manipulated by Dreadlords, although the exact chronology on when is unknown.
It was also a very weird story. I mean, you had the Night Elves sending an army across the world to investigate reckless arcane magic use in Quel’Thalas after ignoring it for 10 millennia in their own backyard (Dire Maul), all while their sacred forests were lousy with demons, satyr, naga, corrupt furbolg, and orcs. I mean, the sentinels couldn’t be bothered to mobilize their army at all during Vanilla (save for a moment in the Alterac Valley BG), but when it comes time to try to justify the Blood Elves going Horde, they conveniently decide to go invade another nation.
The Quel’Thalas 1-20 questline isn’t exactly the best written. It’s canon, but that doesn’t make it a triumph of storytelling. Blizzard spent more time focusing on why the Blood Elves should hate the Alliance and kill every Human, Night Elf, and Dwarf as the game’s PvP system demands, rather than building any reason for them to actually join the Horde. The Blood Elves all but instantly trust the Forsaken whom pinky swear (those whom still had pinky fingers) that they weren’t bad or like the Scourge at all. Meanwhile we saw nothing between the Blood Elves and Orcs, Tauren, or Trolls. No attempts to overcome old hatreds.
That’s been a consistent problem with the Horde narrative where the Blood Elves are concerned as well. Old hatreds need to be ignored and brushed under the rug, while old camaraderie with the Alliance’s races and nations needs to be brushed under the rug until the Horde needs access to an Alliance themed neutral organization (Dalaran), at which point the narrative leans into it. The Blood Elf narrative also needs to ignore recent actions the Alliance has taken on their behalf (restoring the Sunwell being the most glaring example).
The Purge of Dalaran is a whole other can of worms. Suffice to say between rewrites, cut content, and bugs, it’s a headache. All I’ll say there is, Garrosh had OG Sylvanas levels of subterfuge and cunning going for him, sabotaging the talks the way he did.
Of course, why those talks were happening at all is another problem with the Blood Elf narrative. Its as if the Devs said, “Okay, yeah, their reasons for joining the Horde were super weak, we just wanted a pretty race with Paladins on the Horde to help them raid more,” and then tried to give them a new reason. Except… as noted, between rewrites, cut content, and bugs, it was more of a massive headache than anything else.
[Edit]: And to re-iterate, I am not saying the Blood Elves could not or should not have gone Horde. I’m saying Blizzard wrote a lousy story behind it, and their attempts to justify it since have been equally lousy. This isn’t all that surprising though. Blizzard’s storytelling hasn’t exactly been known for quality.