they don't have to agree on anything, the warsong already took the place and had settle on it, they didn't leave, thats why the night elves broke the deal and attack then.
the exact Same thing happened in wtlk, night elves break the agreement and attack the orcs using wrathgate as excuse.
Again, the warsong clan was already settled there, and they never left.And no, there were no "parts" of it. During Cata they got "parts", before that it wasnt a deal.
The warsong was/is the biggest and stronger clan within the horde, they deserve their praise, and regardless of what thrall think about Grom Garrosh was one of the best options in a moment of war.And Thrall is notorious for his strange obsession with "Hellscream's legacy", those being Warsong and Garrosh, both of which he tried to elevate much higher then they deserved and against the better judgement. Even against Garroshe's own protests since he just wanted to "make it up" for Grommash by making his kid a Warchief even though he wasnt ready.
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Why the horde and alliance getting their ass kicked in broken shore is the reason the conflict re-start?
From everything we know, the "war" never stop at SoO, they end in an cease-fire, and Gen actions are definitely considered as breaking the cease-fire and re-ignite the war, but they ignored that plot to make the horde the villains with a shit ass excuse
I mean, Sylvanas herself said that in the book, she used Gen attack as excuse that the alliance was never going to stop attacking then, so they had to attack first.