First 7.2 mage quest Aethas tells you someone let the Legion into Dalaran. If you ask him to explain, he tells you this.
I mean this was bound to happen. Of course they can't be trusted. Should've killed them all when we had the chance, along with the rest of the Horde scum.
Inb4 "Alliance and Horde need to unite to defeat the Legion"
What's the point of uniting with people who will end up cooperating with the Legion?