With the decline of the Brotherhood of the Horse and the loss of the Alliance Expeditionary Force on Draenor, a vacuum was created. Whether or not nature abhors such (the Great Dark Beyond is almost entirely vacuum, after all), the people of Azeroth have often shown that they cannot abide a role abandoned. Into the gap stepped the 7th Legion.
The Brotherhood was a knightly order, but the Legion is not. What is it? Well, best to let it speak for itself.
The Kor'kron, their direct Horde counterparts, grew out of the Warchief's will and were created to be an elite. The 7th Legion became elite. Standard-bearers for the new Alliance, their feet have trodden the ground of every major conflict, battled demons, Silithid and Undead and today serve to safeguard the Alliance in every theater of war. From the lowliest Alliance solider to the mightiest warriors, the 7th Legion stands apart.
By the Light and by the might of the Alliance
Unlike the Kor'kron, the 7th Legion did not see much action in Outland, as the heroes of the Alliance Expediton were already in force. The Legion was born out of the men and women who accompanied Jaina Proudmoore to Kalimdor following the near-total destruction of Lordaeron by the former Prince Arthas Menethil at the behest of his master, the Lich King.
Lordaeron became a howling wilderness of corpses and demons. Many of the survivors fled south, but others went west with Lady Proudmoore and her loyal followers from Kul Tiras, as well as others such as the Gilnean Brigade. These disparate Human and Dwarven forces united under Lady Proudmoore and served with distinction on the slopes of Mount Hyjal, taking the first and heaviest brunt of the fighting against the demons and undead forces of Archimonde the Defiler.
The victory of the mortal races against odds so vast as to almost be ludicrous was merely the first step in the foundation of the Legion proper. The Human nation of Theramore saw itself nearly destroyed during the battles that resulted from the arrival of Admiral Daelin Proudmoore and his assumption of power (the campaign known today as the Foundation of Durotar). Clearly, as the most experienced forces then in existence, the 7th Legion most likely took part in the fighting. Ironically, their defeat would derive from Jaina Proudmoore's decision to support the fledgling Horde over her own father. His death ended the conflict for a time.
Eventually, Theramore learned that rather than all humanity's being destroyed by Arthas and the Scourge, many of the people of Lordaeron had escaped and sought refuge to the lands south of the former Kingdom. Those who didn't settle in Southshore (a region of the former Alterac Kingdom) made their way to the lands of the Kingdom of Stormwind, where King Varian Wrynn took them in just as they had once done for him.
With the resumption of contact between Theramore and Stormwind came a new Alliance, as the Dwarves, Gnomes and Humans of the Eastern Kingdoms allied with the mysterious Night Elves and Humans of Theramore. Together, these allies made formal what was already fact, and each supplied women and men to the front line fighting force of their combined armies. The High Elves who had traveled with Jaina to Theramore also contributed more than their share of battle mages to the 7th, giving it unparalleled flexibility as it could draw from each race's strengths to fill its ranks.
When the Silithid began their return the Alliance mobilized their 7th Legion to take part in the joint operation with the Horde known as the Might of Kalimdor. It was the second time the Legion took part in such a joint mission, even allowing themselves to be commanded by Varok Saurfang, the infamous Orc warlord. This period of cooperation, four years after the attack of the Burning Legion on Hyjal, was the last that the 7th Legion would experience.
The next major operation for the 7th came when King Varian Wrynn, recently returned to his position as ruler of Stormwind, mobilized them for service in Northrend following the Lich King's attack on the city. The 7th spearheaded the Alliance efforts in the Dragonblight, High Commander Halford Wyrmbane placing his forces at the discretion of Bolvar Fordragonm Supreme Commander of the Alliance Forces in Northrend. The 7th managed to destroy the Lich Thel'zan the Duskbringer and stymie he forces of his master Kel'Thuzad from the necropolis of Naxxramas. Sadly, their involvement on this front meant that when Highlord Bolvar and his forces were ambushed by Forsaken at the Wrathgate, the 7th was unavailable to save him.
Following the defeat of the Lich King, the 7th Legion have been seen slowing the Forsaken advance into Gilneas under High Commander Wyrmbane. 7th Legion forces composed of every race in the Alliance have now fought on every continent. Considering that some members of the original 7th Legion were almost certainly Gilneas Brigade forces, in a strange way, the Legion's battle in Gilneas was a belated and war-torn homecoming for a force born out of Jaina's flight from the plague of undeath.
It has now been 10 years since the 7th was founded. They've fought every kind of foe currently menacing the Alliance, in every battleground on Azeroth, and have done this not as knights or lords but simply as the women and men who pledged themselves for their people's sake, in their people's name. There are other fighting forces you might send, but none can ever say that they would fight harder than the 7th. They are not knights. They are not the hand-picked honor guard of a leader. They are simply the best, picked by survival, proven by battle after battle, the sons and daughters of the Alliance. The first strike belongs to them. The last strike is all their enemies see.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.