In Tides of War, The Horde murder some of the blue dragon flight to steal the focusing iris and craft it into a bomb capable of destroying Dustwallow Marsh completely.
The Kirin’tor use this as a justification to stand against The Horde and defend Theramore against their attack, which sadly is where Rhonin gives his life to contain the mana bombs radius.
They voted to allow the Horde back in to stand against the Burning Legion when they invaded in Legion.
Because Blizzard always make the greatest Alliance heroes to go neutral because Khadgar can single handed wipe the Horde, even tho Alleria, Turalyon and Danath who are Khadgar best friends actually stayed Alliance… so pretty much a Blizz excuse to not 1 shot their Beloved Horde, that’s why The Vindicaar is never used, Malfurion pretty much wasted, Jaina was ok but held back and I can keep going.
Ah I see. That quest was removed after pre-patch it seems. Very unfortunate, that seems like a crucial bit of lore.
Because Chadgar is too OP for the horde.
Because this is WoW and therefore realism doesn’t enter the picture very much.
Why would the Argent Crusade tolerate the Horde re-plaguing the Plaguelands?
Why would the Cenarion Circle tolerate the Horde burning a world tree?
Why would the Kirin Tor tolerate the Horde killing their mages in Silverpine and Theramore?
Why would the Alliance forgive the Horde after war and genocide?
Because Blizz can’t write.