I see now why your Alliance and your Horde cannot stop fighting. Every reprisal is itself an act of aggression, and every act of aggression triggers immediate reprisal.
~ Taran Zhu
In World of Warcraft there is no good and evil side when it comes to the ongoing conflict between the Alliance and the Horde, they both think of each other as evil and their side as good (due to Demonization and acts of aggression by rogue members of both factions). Both factions are either at war with each other, engaged in a cold war against eachother, or under an uneasy peace treaty that can be broken at anytime.

The Alliance think of the Horde as brutal barbarians who would ravage and destroy their lands due to the acts of corrupt Orcs and Undead - (in the original Warcraft mythos Orcs and Humans were bitter enemies and despite new leadership they still distrust one another due to prejudice and injustice on both sides).

The Horde think the Alliance are arrogant tyrants who consider themselves superior and the the Horde are sub creatures, the Horde worry that the Alliance would hunt them down and destroy them.

The truth is they're both right, certain members from the Alliance and Horde races have wronged each other and because of that there has been mistrust between them, blaming all for the actions of a few. The problem between them is racism.

The only thing that can bring peace between these two factions is the Burning Legion.

Alliance as villains

  1. Humans put Orcs in concentration camps.
  2. Humans abandoned a group of Blood Elves to the Scourge to die.
  3. Humans chased away the Forsaken when they tried to contact their families.
  4. Dwarves mined in Tauren land without permission.
  5. Humans sank a Goblin slave ship killing many innocent Goblins.
  6. Tauren are considered savages by many Humans and Night Elves.
  7. Thrall himself was a prisoner for many years in a human concentration camp, despite this he grew to be neutral and did not seek bloodshed between the races - however he was no fool and (rightfully) distrusted the Alliance after what they did to him.
  8. An army of Humans and Dwarves destroyed a peaceful Tauren village in the Barrens and killed every Tauren inhabiting it, even the children.
  9. Night Elves and Cenarius try to kill Grom Hellscream and warsong. Without trying to talk first.
  10. Worgen try to invade the territory of the forsaken.
  11. Jaina proudmoore purges Dalaran of the Sunreavers.
  12. Night Elves loathe Blood Elves as they are an offshoot of High Elves, who seceded from Night Elven society, the Night Elves viewed sedition as an act of treason, and Night Elves never forgive or forget.

Horde as villains

  1. Hellscream was a tyrant who tried to rule Azeroth, he was a tyrant amongst the Horde and he kidnapped Pandaren cubs to force neutral Pandaren to join the Horde.  In the end, both the Alliance and the Horde work together to defeat him. 
  2. Demon-corrupted Orcs destroyed many Human cities.
  3. Orcs killed many Draenei during their bloodlust.
  4. The Forsaken invaded Gilneas.
  5. Many Goblins are known to be greedy when it comes to money; many would do any thing when come to wealth no matter how wicked.
  6. Some Blood Elves worked to destroy the Draenei.
  7. Hellscream had the Sunreavers steal the Divine Bell, and because Jaina was still traumatized by the destruction of Theramore, completely destroyed Jainna's hope for peace and made her hate the Horde and caused her to purge Dalaran of the Horde influence.

Working Together

Despite their hostilities, both sides have worked together amongst greater threats, such as The Burning Legion, The Lich King, Deathwing, and Garrosh Hellscream.  Both sides have also shown signs of respect for one another and both sides have made attempts to peace. By Warlords of Draenor, the Alliance and Horde still have some tension but by now it's mostly a cold war status. However, aside from skirmishes (PVP), both sides are working together to bring down the Iron Horde.

Alliance's respect for the Horde

  1. After Garrosh was defeated and Vol'jin became the new warchief, Varian acknowledged that, while the Horde committed vicious crimes, several members stood up against Garrosh, so Varian said he was willing to end the bloodshed but warned he would "end" the Horde should they ever cause trouble again.
  2. Jaina told Thrall that he's respected, even amongst the Alliance.
  3. Anduin believes that the Horde is good, along with the Alliance. He and Tauren Chieftan Baine Bloodhoofe share a close friendship, with Anduin giving Baine the mace Fearbreaker as a gift.
  4. After Alliance players defeat Dranosh Saurfang, Varok asks to see his son.  Varian allows Varok to claim Dranosh's body and commented that Dranosh was a hero at the Wrathgate and deserved a hero's burial.
  5. Field Marshal Brock in Hellfire Peninsula gives a quest called Enemy of my Enemy.  The objective is to destroy three Fel Cannon MK1s.  The reason is because they are aimed at the Horde outpost, Thrallmar and Brock admitted the Alliance needs the Horde to help hold the line at the Dark Portal.
  6. After Taran Zhu mediates between Lor'themar and Jaina after Shan Bu's death, Jaina says no true peace between the Horde and Alliance would exist as long as Garrosh was Warchief. Jaina's expression softens when Lor'themar says that why they should save their strength. Jaina then asks her army to fully back down and both Lor'themar and Jaina bow to each other.

Horde's respect for the Alliance

  1. Baine and Anduin became friends and thus the Tauren protected the young prince.
  2. Some members of the Horde were disgusted by the destrucion of Theramore.
  3. Thrall has limited respect for the Alliance, even if his past means he can't trust them fully - he is at least willing to try and co-exist with them and doesn't hate them fully. (which is more than can be said for Garrosh or other radicals in the Horde).
  4. During Patch 5.3 Escalation, Vol'jin accepted help from the Alliance to obtain supplies from the Kor'kron.
  5. The Horde tried to assist the Alliance during the Wrathgate attack.
  6. The Darkspear Trolls went to Stormwind to warn the Alliance about the plans of the Zandalari.
  7. During Garrosh's reign, the Blood Elves considered rejoining the Alliance.  When Jaina kicked the Sunreavers out of Dalaran, the Blood Elves held Garrosh just as responsible for their exile as much as they held Jaina responsible.