Once, the orcs were an honorable tribal race who were as one with nature. But the orc homeworld Draenor had become a red, desolate wasteland, due to the demonic corruption accidentally discovered by the orc shaman Ner'zhul. But this corruption was ultimately inflicted on the orcs by Ner'zhul's apprentice - the crafty orc warlock Gul'dan, and his demon collaborators.
Their demonic corruption turned their skin from healthy brown to sickly green, made their eyes glow like red fire, and encouraged them to go to war against other innocent peoples of Draenor.
A cloaked human wizard (possessed by the greatest of the demons - Sargeras) on the human world of Azeroth showed visions of Azeroth to Gul'dan, a world ripe for conquest, also tempting him to find and unlock great demonic power within a tomb beneath Azeroth's oceans. In the tomb, Sargeras waited to be unleashed.
Collaborating in secret with the cloaked human wizard, Gul'dan oversaw the construction of the Dark Portal.
Durotan, chief of the noble Frostwolf clan, and his pregnant wife were some of the few orcs who didn't like all the corruption and senseless conflict, and just wanted to get by.
But the orcs were forced by circumstances, and by Gul'dan's plans - to invade Azeroth through the Dark Portal, to pillage and raid.
Durotan's friend, the brave and skilled orc warrior Orgrim Doomhammer didn't really like all the corruption either, but had to look after himself and try to make a difference within the orcish Horde, rather than as an outsider. The orcs did need resources to survive, but they were also being influenced by the demonic curse to be ferocious and bloodthirsty. Not only that, but they were also being manipulated and forced by Gul'dan and his political figure-head Blackhand The Destroyer - the new ruthless titanic War Chief of the orcs.
The orcs poured through the Portal, established their base in the grim Black Rock Mountain, and attacked under the rule of Blackhand. But they had been met with more resistance than they expected, particularly from the human knights on horses.
Durotan eventually spoke out against Gul'dan, and as result, he and his clan were banished. For the most part, the humans did not take kindly to the orcs, seeing them as hideous monsters to be slain. Most of the orcs had nothing but animosity for the humans as well.
Meanwhile, Khadgar, a talented aspiring young human wizard, was sent by the wizard-guild Kirin Tor from their city of Dalaran, to be the apprentice of (but also to investigate) the mysterious powerful human wizard Medivh, who kept to himself in his secluded tower. In Medivh's tower, Khadgar met the half-orc woman Garona, the product of a forced breeding experiment conducted by Gul'dan. Gul'dan had sent Garona both as a messenger to Medivh, and also to spy on the prosperous human kingdom of Stormwind, but she became Medivh's secret lover, and also became friends with Stormwind's King Llane Wrynn - so she struggled whether to side with orcs or humans, especially since her master Gul'dan was so evil and mean anyway.
Elsewhere, Durotan met with Sir Anduin Lothar, devoted knight of the human kingdom Stormwind. Each of them could see that the other had similar sensible values, and wished to avoid war. But the battles between the stranded orcs and the humans of Stormwind happened anyway.
Later Durotan warned his friend Orgrim Doomhammer that things were not as they should be and that Gul'dan didn't actually have the orcs' best interests at heart. Orgrim agreed, and planned to do something about it. But Gul'dan's assassins killed Durotan and his wife, leaving their newborn son Thrall to die in the wilderness (unless, maybe, a human lord finds him and raises him later on).
Orgrim, already fed up with the way things were going, and now that his old friend Durotan had been murdered, finally stormed off to challenge the orc War Chief Blackhand. Orgrim killed Blackhand and presented his severed head to the Horde, becoming the new War Chief.
Eventually Khadgar and Garona discovered to their horror that it was really Medivh who had opened the Portal which let the orcs into the human world of Azeroth(he was the human wizard who'd worked with Gul'dan), and that Medivh was actually being possessed by the great demon Sargeras, and had been since birth, though his good human side had tried to resist. Khadgar and the knight Lothar reluctantly stopped Medivh by killing him, thwarting Sargeras' master plan, knocking Gul'dan, who had been plundering Medivh's mind for information remotely, into a coma.
Orgrim decided he had to lead the orcs to the final battle and finish off the war. The humans defended their domain valiantly, but the orcs were too powerful, and Stormwind city was conquered. The half-orc Garona was forced by her master Gul'dan's sorcery to assassinate King Llane Wrynn of Stormwind, even though they had been friends. King Llane's body was toppled over the castle walls into the moat, and Stormwind was reduced to burning rubble.
The knight Lothar realized that there was no hope left there, and set sail with the wizard Khadgar and the survivors of Stormwind to the north - to warn and seek the help of the human kingdom of Lordaeron.
After the battle, Orgrim sought out Garona the half-orc, having heard that she was Gul'dan's agent, and tortured her until she revealed where Gul'dan was.
Orgrim went to Gul'dan's hideout and slaughtered his warlocks, coming at last to Gul'dan, who bowed down and begged for his life. Orgrim let him live, even though he hated him, because he might prove useful for Orgrim's new Horde. Orgrim would do what he had to, to ensure the survival of his once-honorable people.