The Horde was doomed from the very moment it's been reformed by Thrall. Let's see who were the members of the vanilla Horde:
- orcs who were prisoners of humans not long ago, so a grudge was still present
- tauren, who had nothing to do with humans or the old alliance before, meaning no grudge was there; their druids were also allies of night elven druids for a long time
- darkspear trolls, who had nothing to do with humans or the old alliance before, meaning no grudge; yes, the jungle trolls have a history with humans, but the darkspear splintered from them to begin anew
- undead, who were shunned by their former human allies, so a grudge was still present
Fast forward many years later, and nothing changed - orcs and undead hold a grudge against the humans, while trolls and tauren are more on a neutral side of things.
Thrall's mistake was taking in all of the orcs, even those who drunk the blood of mannoroth again in ashenvale and those who were still holding a grudge, and accepting the forsaken into the horde. With these two actions, he made the peace with the alliance impossible.
Cata zones perfectly showed how he failed in securing the peace. Orcs jumped into the war train the moment Garrosh was in charge, and Sylvanas was thrilled to have a legit reason to kill humans. I don't know if Thrall's intentions were for the horde to live in peace. It they were, he failed at the very foundations; if they weren't, I don't know why he was surprised.