Originally Posted by
Super Dickmann
The war started because of Varian's reaction to what he saw in the Undercity, and his experience in the slave pits under Rehgar. He was under no illusion that Sylvanas or Thrall directly did the Wrathgate and pinned it on Putress. But he did view it as them having enabled Putress and the experiments he saw going on that were just repurposed rather than started by Varimathras affirmed this to him. The slaver who owned him and put him in gladiatorial fights was a top advisor for Thrall after all and this was proof to Varian that Thrall was negligent to the point of harm and it was time for war. So he declared it, then got ported out. From then on, there's a state of war, hence the PvP objectives in Grizzly Hills, Icecrown and the gunship battle. The peace summit at Theramore failed. Honor's Stand, like you mention, preceded the Cataclysm and was initiated by the Alliance. The only things to happen in Ashenvale before the Shattering were false flags by the Twilight's Hammer.
The counterargument here is that Garrosh was going to start the war even if none of this was the case. This is true. War was inevitable even if Varian and the Alliance hadn't started it first because of Thrall's poor leadership choices, placing the orcs in a desert out of racial guilt where they were suffering from a lack of food and resources. They were not self-sufficient because of their inhospitable territory and were reliant on two things - imports from the night elves and an Alliance that was too passive to take advantage of this. By the time of Wrath, Varian was in charge and the Alliance was active again and with the false flag, imports were cut. This along with the Shattering exacerbated the existing orcish deprivation and discontent and per Garrosh's leadership story are what motivated his war. For the orcs to last, they needed more resources and Garrosh was rightly willing to go to war to ensure they had it.
tl;dr Varian started the war in Wrath, no peace effort got anywhere, Alliance hit first before the Cataclysm, Garrosh would have started it anyway though because of what the orcs needed. Thrall's shit leadership is at fault either way, since it's the motivator for both Varian and Garrosh.
P.S: If the Wrathgate retcon is true, then the above still applies, since nobody knew that Sylvanas was a mouthbreathing retard bombing her own side for no reason when she can aim the catapults at Arthas, they've shown the ability to do so and the power to kill him and it was the sole reason for her existence whereas killing the Horde and Alliance didn't gain her anything. Since it was not a factor in anyone's decision-making, it has no bearing on the grounds for the war.