It could well be one of the factors that the alliance want to war with the horde because of some of the things the forsaken have done, but likewise, the forsaken have zero dam reason to trust anything humans want, and are just as just in pushing the humans from there lands.
The alliance claiming lordearon belongs to them, when it really belongs to the forsaken, yeah, thats reason enough to want to strike back against the humans.
Also, you try and make it sound like the forsaken being the sole reason for the war carrying on, your really just not paying enough attention to detail. The orcs faught humans, the humans captured and made slaves of orcs, the orcs faught back and won there freedom, the orcs left those lands and travelled to kalimdor, the humans followed and tried to wipe them out, the orcs faught back, the orcs tried to build in there new lands, the night elves attacked them for cutting trees down, the orcas faught back... and so on and so on.
I'm sorry, you might not like this, but there is ALWAYS a reason for both sides to go to war, and it IS NOT just on the bases of what one group did, there is ALWAYS a two sided coin. Deal With It.