There has only been 1 alliance. There isn't some old alliance and new alliance there has only been the alliance. This is from Blizzard themselves.
In TFT Garithos was the ruler of the Alliance he was in charge and therefore his actions were the actions of the alliance. Even then the Blood elves were willing to go back to the Alliance as their first choice. But there the ambassador meant to negotiate with them was willing along with Night Elves basically have them all killed off by sabotaging the Sin'dorei biggest defense against the scourge, without which could easily have the Sin'dorei overrun. Not forgetting Dalaran doing nothing to help their allies when Garithos was trying kill them all for the gall of not dieing on a suicide mission. That's 3 betrayals from Alliance BEFORE the Sin'dorei joined the horde.
Then there is the purge of Dalaran in which Jaina and her lapdog went to kill anything with green eyes and long ears because Jaina doesn't understand how to be rational.