I mean, even ignoring things like other clashes with humans and the fact that by the time of Invasion of Gilneas the Alliance-Horde war has already began, given how Sylvanas sent envoys all over the world after she created her kingdom back in W3, yeah, she most likely has tried building up diplomatic relationships with the Alliance. And for
some reason, she joined the Horde even though she thought lowly of it and didn't like Orcs. Also, if you want to argue from the point of relationships with not the Alliance but humanity in general, one of the biggest threat to them in their early years was the Scarlet Crusade.
And Forsaken are in the Horde only because Horde book-keeper made a mistake in the census once and no one bothered to fix it since then. Same applies to Blood Elves. Hell, even going back, the Forsaken rallied behind her because each of them tossed a coin to decide and they all flipped the same side. The non-Forsaken that helped them during the Civil War in the Plaguelands just happened to be there in the Forsaken-Scourge battles and didn't attack the Forsaken by accident. Plus, as per the post by @
Venziir/@Zulkhan, Alliance isn't exactly pristine when it comes to diplomatic solutions when dealing with the Forsaken.
They literally meet with each other and make a secret pact.
Except for the part where Alliance suddenly breaks the ceasefire and kills a ton of Forsaken which forced Sylvanas to intervene herself and bring reinforcements.
Gosh, stop validating each other in this TEH CIRCLEJERK.
Yeah, no. The only confirmed case of Blight having such an effect is Southshore and the strain used there was stronger than the Forsaken thought it was. Given how there are no living in Forsaken Lordaeron and how Forsaken are still subject to true death, inheritance can work just fine. Even if it didn't, this tangent is irrelevant from the perspective of evilness. And given how they don't need agriculture, how is agriculture supposed to be critical for their economy? And Forsaken are expansionist because they want the protection of the natural borders. Also, pretty much every other nation on the continent has already collapsed, so they are simply filling the vacuum.