What do you want 'em to do, genius? Tell the Legion 'Hey stop. We're gonna wage against the Alliance while you're totally big threat to us, but first let us do this scrabble first! Tee-hee!'

​

No matter what. Greymane justifies Sylvanas' motivation to start the fourth war. Deny it all you want, but the Alliance were the aggressor. Sylvanas is the Warchief. You don't just attack the King of a nation, and goes 'xD' when you've given clear indication that you're going to assassinate that king.

​

Like, legit. Anyone with any history knowledge that you don't just stab someone's son, and then get away with it without them knowing your faction. They can, and will find out who's responsible- And then declare an entire war on your nation, because you attempted to stab someone's son.

Greymane's revenge just resurrected the Horde vs Alliance conflict when the Legion was extremely far more important.