I'm just going to leave some pointers too.
- Varian wanted any excuse to go to war with the horde, even when its current warchief and the alliances acting peace negotiator tried to settle things
- He attended a summit Jaina arranged, and at a moments notice he attacked Thrall because of seeing someone he had no evidence of being associated with the horde there (Garona, the half orc who killed his father), and showed Garrosh humans can't be trusted.
- He further pushed this when the meeting in dalaran occurred and he egged Garrosh on to attack him, leading Garrosh to further see humans as scum that needs to be wiped out.
- He came to undercity with Jaina to fight against those that caused the wrath gate, and attacked Thrall, which owrd obviously got back to Garrosh.
At every turn in wrath, Varian made Garrosh see humans are scum who will attack orcs without thought or honor. Accusing anyone else of starting this war, even Garrosh himself, who is the biggest warmonger currently in warcraft, only started it because he was provoked by the alliances ruler.