Yes, it is. You just don’t know the lore because you don’t read anything.
As far as the Alliance declaring war, it happened in Wrath of the Lich King:
They have lost the Undercity? Then the time to strike is now. We will deal with Putress ourselves and retake the Ruins of Lordaeron for the Alliance!
–A Royal Coup
Here we see Varian justify a revanchist invasion of Horde territory when he’s under the impression the Horde lost it.
King Varian Wrynn says: THRALL! HERE?
King Varian Wrynn says: ONWARD! We end this now!
…
King Varian Wrynn says: I was away for too long. My absence cost us the lives of some of our greatest heroes. Trash like you and this evil witch were allowed to roam free – unchecked.
King Varian Wrynn says: The time has come to make things right. To disband your treacherous kingdom of murderers and thieves. Putress was the first strike. Many more will come.
King Varian Wrynn says: I’ve waited a long time for this, Thrall. For every time I was thrown into one of your damned arenas… for every time I killed a green-skinned aberration like you… I could only think of one thing.
King Varian Wrynn says: What our world could be without you and your twisted Horde… It ends now, Warchief.
King Varian Wrynn says: ATTACK! FOR STORMWIND! FOR BOLVAR! FOR THE ALLIANCE!
For too long have the Horde been left unchecked. We allowed their territories to prosper and in return for our generosity they plotted and planned our demise.
Peace? Useless… it’s gotten us nowhere. We have lost some of our greatest heroes to “peace.” Let us see what battle brings.
–The Battle For the Undercity (Alliance)
When alerted to the presence of Thrall & Sylvanas, Varian, despite KNOWING that the Horde was innocent of the Wrathgate, decides to attack them with the aim of decapitating Horde leadership and abolishing the Horde outright.
As far as the ceasefire goes:
Garrosh referred to a treaty between the Horde and the Alliance, signed shortly after the fall of the Lich King. Both factions had been sorely damaged by the long battle, and both sides had agreed to a cessation of hostilities, including the struggles at Alterac Valley, Arathi Basin, and Warsong Gulch, for a brief time.
–“The Shattering,” p19
And do note that this excerpt from “The Shattering” not only is evidence of the ceasefire, but further reinforces the above quest text from Wrath of the Lich King about the Alliance starting the war.
As to the question of “who broke the ceasefire?” It’s actually very simple. We know for a fact that Garrosh’s first major offensive against the Alliance was in Ashenvale, as that’s where his war aims are centered. We see that in “Wolfheart;” however we ALSO know from “Wolfheart” that Garrosh’s Ashenvale offensive begins AFTER the Cataclysm. And yet, we see that the Alliance breaks the ceasefire and invades Horde territory again BEFORE the Cataclysm:
(Gossip) Kilrok, how did we lose Honor’s Stand?
It was a clever ruse, . Alliance forces streamed out of Northwatch and marched up the gold road, besieging Crossroads as dusk fell. Then, in the heart of the night, they force-marched southward, leaving their campfires burning behind.
Our meager forces at Honor’s Stand were caught unawares as the Alliance poured through the pass at dawn’s first light. Many of our bravest warriors fell before their blades.
(Gossip) This was before the Cataclysm?
Yes. We regrouped east of the pass and prepared a counter-attack, but the next day, the skies reddened and the land was cleaved in two, as easily as a child might split open a cactus apple. See how the devastation stretches from horizon to horizon?
–Kilrok Gorehammer
