I feel for Aethas, in this sense. I sympathize with his impulse toward reconciliation. Why shouldn't he have tried? History was on his side. Quel'Thalas and Dalaran have, in fact, been traditional allies for quite a long time, much longer than they have been enemies. I don't think his hope was unreasonable, but it did turn out to be impossible. There is just too much bad blood - on many sides - for any sort of truce to exist at this point, and it will likely be a very long time before those wounds are healed enough for bridges to be rebuilt. Sometimes compromise just isn't an option.
Fair warning: there are minor spoilers for patch 5.3 below the cut.
As we move into the realm of patch 5.3, we're seeing more of this, and more of the fallout when painful loyalties are tested too far. As multiple characters continue to tell us, the Horde is tearing itself apart. Some of the Horde's most staunch members: Vol'jin and Baine, son of Cairne, are at last drawing the line beyond which their loyalties will not hold. They've been eying that line for a while now, likely knowing this decision was coming. It does not come without its price. Remember the Blood Oath of the Horde?
Lok'tar ogar! Victory or death - it is these words that bind me to the Horde. For they are the most sacred and fundamental of truths to any warrior of the Horde.
I give my flesh and blood freely to the Warchief. I am the instrument of my Warchief's desire. I am a weapon of my Warchief's command.
From this moment until the end of days I live and die - For the Horde!
Except that's a much easier manifesto to accept as philosophy than reality, and in reality, you can only push so far, so hard, for so long. This isn't the first time a Warchief has tested the limits of this oath and seen it break. Loyalty is not infinite and anyone who treats it as though it is will eventually exhaust its supply.
The tug of war between painful loyalties is a theme that crops up regularly in WoW, particularly (but not exclusively) among the members of the Horde, and it's no surprise that it does. The Blood Oath of the Horde's existence itself assures that it will, because any Warchief who takes it to its logical conclusion will find the hard way that their subjects are not automaton soldiers happy to throw their lives away at the Warchief's every whim. The absolutism of the Blood Oath cements its own inevitable failure, because that oath leaves no room for acknowledgement that the citizens of the Horde are individuals with their own agendas, desires, and personal loyalties. Individuals will adhere to their leaders' whims up to a point, and beyond that point lies rebellion.
One possible way to deal with that is to get a sense for where that point lies for the majority of people, and just never push beyond it. Thrall's tenure as Warchief is basically an example of this. Thrall didn't really change the structure of the Horde - the Blood Oath still exists, after all - and there were many people unhappy with his leadership, but not enough to tip the scales into revolt territory. That tactic can work, but it's dependent on having a Warchief savvy enough to employ it, and, as the WoW narrative has been showing us for a while now, not all Warchiefs are.
A better way, for the long term health of the Horde, is probably to do away with the concept of the Blood Oath entirely. Loyalties will always be in conflict, to an extent. Someone like Lor'themar is always going to be balancing his loyalty to the nation he leads against his loyalty to his allies, but bear in mind he sought allies solely in order to further protect his people. It should be obvious that when push comes to shove, he's going to choose his nation over his allies. The same can be said of Baine, and Vol'jin, and the other Horde leaders. And why shouldn't they? All of them came to the Horde already sovereign leaders in their own right. They are not Garrosh's officers, and they are certainly not his friends.
And it is precisely that lack which results in the Alliance's remarkable internal stability, despite the vast, disparate nations that comprise it. The Alliance does not ask its members to choose between their loyalty to their nations and their loyalty to their allies, and thus ensures that mutual loyalty is never in doubt. Garrosh, meanwhile, is reaping the rewards of the Tarkin Doctrine in WoW: the more you tighten your grip, the more they will slip through your fingers.
The next Warchief, whoever that may be, would do well to take a good, hard look at why the Horde's internal loyalties have divided and failed. Directly, it can be attributed to Garrosh's totalitarian leadership, but underlying that is the system that grants him the option of being a totalitarian leader at all. The Horde, in coming through the next few patches, could do worse than to take inspiration from their blue counterparts. They could do worse than to become more like the Alliance.