They were mostly the greymane forces with some reinforced by the King Anduin but he tell greymane to engage in combat only if the forsaken attack him first which he actually do, also with the tension with the death of varian and blaming the horde, yeah both have most to lose if they kill one of them
Yeah that is what i say, genn atacked first, but they soldiers even the 7 legion have to fight for survive and maybe capturing sylvanas for them can maybe negociate some kind of ammisticia or a little the peace, even the dwarf we kill if you do the horde version just want to capture her.
Yeah they attacked first for FAULT of greymane and they are already marked as enemies to kill at spot even nathanos sends you to kill the alliance forces for that, they want it or not have to fight but if you think about that the only one who want to kill one of the other was Nathanos and even if he kill greymane the forsaken can just using him as the scapegoat