While with teh Alliance their major forces are made of Humans and Nighelves, who have a strong interest in puting the Horde down aswell.
That being said I would be surprised if the Forkasen would realy play a role in the Siege, I am pretty sure they will stand outside watching what happens.
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The point of the Alliance existing, and having an army, is not to humiliate orcs and grandstand about their moral superiority. It's to secure geopolitical influence and power.
To answer your question, the alliance could flex by enforcing an Alliance 'peace keeping' force (along with permanent military bases in Durotar) upon the divided and vulnerable Horde. Nobody is saying the Alliance need kill and burn everything; but if they are in Orgrimmar with an upper hand... then yes, it's retarded for them to 'turn the other cheek'.
As a Horde myself I'd prefer resisting an Alliance occupation, than working with the Alliance, high fiving them, and watching them leave us to regroup and meet them on equal terms later.
People may want to deny it, but this game, this (expanded) universe, will always be Orcs vs. Humans.
This wouldn't work in the slightest, having soldiers stand guard in Orgrimmar, having political control to some extent, yes. But not a permanent military base in the homeland of the Orcs.
"Why do we fight? To protect home and family, to preserve balance and bring harmony. For my kind the true question is, what is worth fighting for?"
The idea is that as long as the Horde is fighting itself, it is drastically less capable at maintaining the stalemate that has existed for so long. The Alliance could and should take advantage.
Other races in large numbers? No. The Orcs have always been the power of the Horde because of their numbers relative to the other races. And many/most Orcs liked the way Garrosh did things. He brought them victory. He brought them the spoils of war. He enabled them to strike back at the hated Alliance.
And its the Orcs who take the brunt of the assault.
With a large number of Orcs dead, with the Horde leadership and government smashed, with the inevitable breakdown of trust that arises from a Civil war, with their elite troops all but wiped out and others almost certain to flee to escape "justice" then yes...the Horde armies got hit very hard.
The good aspect of it - the Horde will end up with a much more equal racial balance.
Of course, it remains to be seen how much of this will be reflected in game.
As it is, the Alliance don't really need to humiliate the Horde or destroy it. That's not in their nature. But they also shouldn't accept continued Horde occupation of their lands, they shouldn't be vindicating Garroshes vision of them and they shouldn't be treated as the losers in a conflict where they emerge much more powerful, gaining nothing.
The least the Alliance should expect is a general Horde withdrawal from Alliance territories, to the WC3 borders. and a shift of focus to the Lordaeron theatre.
He who sees his own doom can better avoid its path. He who sees the doom of others can deliver it.
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I want an unconditional surrender from the horde. AKA: A return of Alliance lands.
It seems that if there will be one more patch. Perhaps 5.5 will be the patch that will tell us what the Alliance will get back and what will be the terms of the Horde surrender.
I will never understand why people get so upset over the perceived slights to their fictional faction in a video game with fictional story lines. You want some real hardship, try cheering for the Chicago Cubs.
OMG 13:37 - Then Jesus said to His disciples, "Cleave unto me, and I shall grant to thee the blessing of eternal salvation."
And His disciples said unto Him, "Can we get Kings instead?"
However, even without it those Blackrocks were still part of the Horde. They were still part of the armies that faced the Alliance.