The Alliance was also divided on the question of Alterac. Following that nation's treacherous alliance with the Horde that allowed Doomhammer to nearly sack Lordaeron, the kingdom of Alterac had been shattered by Alliance forces and its rulers deposed. This left Alterac a nation in name only, a lawless, broken rump state in need of stability. Was it to be annexed into neighboring Gilneas or Stromgarde? Thoras Trollbane of Stromgarde was certainly in favor of annexing his neighbor, or carving it up between the other Alliance nations. The debates between the leaders of the Alliance nations grew ever more heated and little was decided between them.
So things stood. The Horde was broken, its troll and goblin allies having returned to their own affairs, the orcs eventually imprisoned in internment camps. This decision was hardly a popular one: while a young Varian Wrynn went along with what his elder monarch and former host Terenas Menethil ultimately wanted, Genn Greymane felt that this was the last straw and pulled his nation of Gilneas out of the Alliance altogether. Similarly, Thoras Trollbane of Stromgarde argued bitterly with Terenas that the orcs were too dangerous to allow to live and also eventually pulled out of the Alliance over this issue. Still, Terenas refused to simply butcher the orcs. This debate and its fallout, however, took years. Before it could be decided, a new threat arose.
While the Alliance was mired in these issues, the remains of the Horde on Draenor was not so idle. As the Horde collapsed, former Gul'dan supporter Teron Gorefiend, also known as the first of the Horde's death knights, made his way back through the Dark Portal and found himself on a world spiraling towards complete destruction, tainted and corrupted by the warlock magics Gul'dan had used to secure his power. If you read the novel Beyond the Dark Portal, you've read how Gorefiend was met at the portal by Ner'zhul, who had found himself effectively leader of the Horde remnants on Draenor itself. Unfortunately Ner'zhul was injured in the backlash when Khadgar destroyed the Dark Portal, and so for two years things remained at a standstill.
We covered some of this in the Ner'zhul KYL of course, but we'll recap and expand pertinent bits now: Ner'zhul and Gorefiend, despite their mutual distrust and distaste for each other, eventually came up with a plan. SInce Draenor was dying, it was necessary for the orcs of the Horde of Draenor (as they now called themselves) to find a new world. Azeroth had proved itself to be someone intransigent in that regard, but they could make use of the dark portal and of various magical artifacts to construct new portals to other worlds, hopefully finding one in the process that would be suitable to their needs. And so, two years after the destruction of the Dark Portal, they launched an audacious plan. Of course, as desperate as they were, only an audacious plan would really have any chance of success. In order for it to work, they had to reopen the portal to Azeroth and make contact with Kilrogg Deadeye, who for two years had managed to evade the Alliance forces with his Bleeding Hollow clan.
Soon, Deadeye, Grom Hellscream and the Warsong clan, and Kargath Bladefist of the Shattered Hand were making strategic raids into Azeroth, attacking Nethergarde Keep, and retrieving magical artifacts for Ner'zhul's use. Before this, however, Ner'zhul's Shadowmoon Clan (led by Dentarg, the Ogre-Mage who was fanatically loyal to the elder shaman) raided the Bonechewers and retrieved the Skull of Gul'dan itself. Ironically, Bonechewers led by Tagar Spinebreaker would lead a raid to the Tomb of Sargeras to retrieve one of the artifacts Ner'zhul required, the Jeweled Scepter of Sargeras.
While the Jeweled Scepter, the Eye of Dalaran, and the Book of Medivh were all necessary components in the ritual to create new portals (presumably the Book of Medivh contained the knowledge Medivh had imparted to Gul'dan in the creation of the original Dark Portal while the Scepter and Eye were both artifacts whose power Ner'zhul could tap, since he himself had long since been abandoned by the spirits of Draenor) it appears Ner'zhul just wanted the Skull of Gul'dan as a symbol of his former apprentice's death, to gloat over. It's also possible that the Skull whispered secrets of demonic magic that Kil'jaeden had vouchsafed to Gul'dan, (Beyond the Dark Portal certainly seems to imply as much) and so helped lead Ner'zhul down the path of corruption and madness he'd long since started on. Eventually, Ner'zhul traded the Skull away to Deathwing, no longer needing it for his purposes.
The raids on Dalaran and New Stormwind (what we now just call Stormwind) for the Eye and Book, respectively, helped galvanize the moribund Alliance into action. In the two years that had passed since Khadgar had destroyed the Dark Portal the fear had always been that the orcs would return, and clearly now they seemed to be doing exactly that. Terenas took premptive action, ordering Turalyon and Khadgar to find out just what, exactly, was going on. If the orcs were invading again, why didn't they try to keep any of the places they were attacking? Hit and run raids like the ones Grom Hellscream and Deadeye were leading simply weren't the way the Horde had operated before, was this just the opening stages in a new invasion or something else entirely? Since these questions were originally forced to the King's attention by Khadgar himself, the mage was a necessary member of this new expedition (and it was a given that he'd probably have gone with them even if he hadn't been included) and Turalyon, as Lothar's lieutenant and successor was the only one imaginable to command it.
Soon other Alliance heroes were assembled... the elves of Quel'thalas represented by Alleria Windrunner, famed for her ability to track and kill orcs, the dwarves by Kurdran Wildhammer of Aerie Peak and his loyal mount Sky'ree, while Danath Trollbane demanded to accompany the expedition to expiate the guilt he felt for having failed to prevent the orcs from opening the Dark Portal again. Danath's men had bravely held the line against Grom Hellscream and the Warsong (as well as death knights serving Gorefiend) so that their leader could warn Khadgar and Nethergarde of the new orcish threat, and Danath took their deaths personally. For her part, Alleria hated the orcs so passionately that, for her, killing an orc was reflexive, as easy as killing an insect, and she had no intention of being left out of the mission. Kurdran, for his part, was eager to go because he'd heard rumors of black dragons aiding the orcs, and he was hoping for a shot at Deathwing himself. Kurdran was nothing if not ambitious in his dragon killing.
These heroes and the forces under their command pursued the orcs (who had never intended to stay on Azeroth at all, not that there was any way for the Alliance to know this) through the Dark Portal. In so doing, they were the first people born on Azeroth to set foot on the Red World, formerly a lush and verdant planet before the rise of the Horde.
Next week, the Alliance Expedition battles the Horde. Heroes apparently die on both sides, and in the end one world is saved at the expense of the other. Who escapes and who perishes, well, that's all part of the story.