Hence the need to bitchslap the night elves and possibly worgen into sense.
The casus belli of the alliance wuld cease the moment Viserys stop the fucked up shit. Then it would be a matter of either diplamancing the alliance or beating Tyrande and Greymane into submission.
Whether Viserys could win against them is less than definite, and the casus belli don't stop just because the Horde stop doing the fucked up shit, only once the Horde has made amends, what the Horde has done cry for Vengeance, the fact they don't add more to the grudges, don't change the fact, that the Horde owe the Alliance a serious debt of blood.
The Alliance already tried forgiving the Horde once it changed leader once, and that got them Sylvanas as an enemy, this time they wont stop, before the Horde has paid the price for their aggression.
And it's not just Genn and Tyrande he would face, Jaina and Kul Tiras would join them as well, and in all likelihood that would mean the whole Alliance join, as new nicer Horde or not, they are going to split in 2, while their enemy is stronger than ever.
The situation is rather like how the Old gods see the Seven, and just like with the Seven an apology and promise not to do it again isn't enough, the Horde must suffer as we have.
So unless Viserys is ready to make the Horde pay crippling reparations, he can't just end the war between the Horde and the Alliance, and kicking Tyrande's and Genns asses wont work, that will just make the rest of the Alliance really commit to taking him out.
The Horde made it's bed, now they must lie in it.