The First and Second Wars and the Birth of the Alliance
One thing to keep in mind is that while the Old Horde existed prior to the First War (since it was the weapon by which Gul'dan completed the extermination of the draenei for Kil'jaeden, his ultimate master) there was no Alliance to oppose it. When Medivh (controlled by Sargeras) and Gul'dan created the first Dark Portal and the Horde stepped through onto Azeroth, there was no consensus of humanity's kingdoms that the Horde was a threat. Indeed, many of those kingdoms (as well as their allies) had no idea the Horde existed. The only kingdom arrayed against them was Stormwind. Despite the fact that the full might of the orcish Horde was arrayed against this single kingdom, the battle between these two forces lasted a solid five years. While we've covered the First War before, it's worth mentioning that the Horde besieged the city twice before it fell, that it was the mounted horsemen of the Brotherhood of the Horse who defeated the orcs the first time, and the Horde only managed to finally defeat Stormwind through the act of the assassin Garona Halforcen, who turned on King Llane and cut out his heart in front of his young son Varian.
The Second War was when the Alliance of Lordaeron itself was born. After the fall of Stormwind, Anduin Lothar took to the seas, leading as many refugees as possible north to the remaining human kingdoms. He begged, cajoled, wheedled and demanded their aid, pointing out that the orcs of the Horde weren't likely to stop at just Stormwind - they wanted the entire world, and unless humanity banded together, they were likely to get it. In the end, even the united forces of Stromgarde, Gilneas, Alterac, Lordaeron, Dalaran, Kul Tiras and what was left of Stormwind would not be enough to stand against the orcs. The aid of two of the three dwarven kingdoms (the Wildhammers of Aerie Peak and the Bronzebeards of Ironforge), the gnomes of Gnomeregan and the high elves of Quel'thalas would all be necessary to finally end the menace of the Horde, along with a little bit of luck as Gul'dan betrayed the new orcish warchief Orgrim Doomhammer out of his lust for power.
The Decline and Decline and Decline and Decline and Fall of the Alliance of Lordaeron
The Kingdom of Alterac was effectively destroyed during the War, as much because of the Alliance itself as the Horde. The Lord of Alterac, Aiden Perenolde, actually sought to betray the Alliance to the Horde, and as such, he was dispossessed and his lands left undefended to be ravaged by the Horde on their way to Lordaeron during the Second War.
Following Alterac's fate, th Alliance really floundered over the question of what to do about the orcs. The high elves of Quel'thalas simply weren't interested in being part of the Alliance in peacetime - they'd barely even wanted to join it to oppose the Horde, and only the twin goads of their oath to come to the aid of the last of the Arathor bloodline and the Horde's alliance with the Amani trolls had gotten them to take part in the Alliance. With Anduin Lothar, last of the Arathor, dead and the Horde defeated, the high elves basically took their leave of the Alliance. This left the Alliance itself most comprised of humans, for the dwarves and gnomes were essentially no longer interested in the affairs of the Alliance following the Second War.
Stormwind was rebuilt by this time, and the young king, King Varian Wrynn, installed. He would never forget Terenas' kindness in allowing him to reside in Lordaeron during the war. But even with Stormwind's backing, Lordaeron found itself unable to hold the Alliance together. Both Gilneas and Stromgarde soon left, reducing the Alliance to Lordaeron itself, Stormwind, Dalaran and Kul Tiras. Stormwind was isolated on the other side of Khaz Modan and the Burning Steppes and Redridge Mountains, Kul Tiras was out in the ocean, and Dalaran was fairly isolated by its own nature, being a city of wizards ruled by a mage's council. Worse, the coming of Daval Prestor only made things worse - even as the Alliance of Lordaeron debated the internment camps and the costs of maintaining Nethergarde Keep, even as the Alliance Expedition traveled through the Dark Portal to make war on the Horde of Draenor, the dragon Deathwing pretended to be a minor human nobleman and potential new ruler of Alterac. He promised unity, and nearly provided it, but when he was revealed as the dragon he was, Daval Prestor ended up exacerbating the strain and helped speed the effecting dissolution of the Alliance.
When the Third War came, the nations that had banded together for the Second War did not manage a similar feat. While the orcs escaped from the internment camps, and slowly prepared to make their way to Kalimdor (thanks to the visions provided to Thrall by the Prophet, Medivh the Last Guardian) the humans were not so unified, and Lordaeron was left to deal with a new threat, a plague of undeath spread by a group called The Cult of the Damned. In time, the paladin Arthas Menethil, Prince of Lordaerson, would fall prey to an obsession that would ultimately damn his soul and leave him a tool. He would strike down his own father Terenas, and in so doing help speed the destruction of Lordaeron, the nation that had been the heart of the Alliance. Ultimately Jaina Proudmoore would leave survivors from Lordaeron to Kalimdor and help defeat the Burning Legion, while the humans of Stormwind would remain untouched by the Scourge. Indeed, many of Lordaeron's people would flee to Stormwind just as the refugees of Stormwind had once fled to Lordaeron.
Following the Third War, the night elves of Kalimdor would join into a new Alliance with the humans, the dwarves of Ironforge, and the gnomes. In time, the draenei would join, followed by the worgen of Gilneas, formerly the human subjects of Genn Greyman (the very king who abandoned the Alliance of Lordaeron all those years ago) and finally a few pandaren explorers as well. This Alliance shares very little with the former Alliance of Lordaeron - for one thing, Lordaeron itself is now gone, its lands ruled by the walking death. The current Alliance is simply that, The Alliance, and it is much more of a collection of peoples rather than the union of human kingdoms its predecessor was. The Alliance has managed to endure for a decade now because it has faced so many implacable enemies, from the 'new' Horde of Thrall and the True Horde of Garrosh to Deathwing, the Lich King, and even an attempt by the Burning Legion to infiltrate Azeroth. Whether the Alliance can maintain its unity in the face of the Iron Horde will be its biggest challenge. Yet so far, it can at least be said that the Alliance is a continuation of its predecessor, and there are no Dark Alliances, Fel Alliances, or Iron Alliances seeking to rival or supplant it.
For good or ill, the Alliance is the Alliance, and it has endured.
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World of Warcraft in WoW Insider's Guide to Warcraft Lore