The true timeline of the First and Second Wars are blurred somewhat. The initial invasion of the Horde was crushed completely by the Humans, but after a change in power the Orcish Horde came back for a second attempt. The Second War begins when the humans attempt to retake the fallen Kingdom.
Prelude to the War
The humans of Stormwind lived in peace and prosperity on the continent of Azeroth for many years. But that ended when the sorceror Medivh - possessed by the Dark TitanSargeras - opened the dimensional gate that later became known as the Dark Portal. Through the Portal came the Orcish Horde (corrupted by Sargeras' lieutenant Kil'Jaeden) seeking to exterminate the humans. The Orcs overconfidently unleashed their first assault directly on Stormwind Keep, but failed to defeat the Human defenders. This attack is considered the start of the First War.
While unconscious, Medivh sends Gul'dan a vision of Azeroth and the vast resources it contains.
Medivh awakens from a six year coma on the same day that Llane becomes Prince of Azeroth, and sometime thereafter begins work on the Dark Portal. The warlocks on Draenor find a small tear in the dimensional fabric and begin studying it, using this information to unite the Orcish Horde once again.
The Dark Portal is opened and the Horde begins their invasion.
Initial battles do not go well for the Horde. Expecting an easy victory against a weak opponent, the orcs rush to assault the fortress of Stormwind Keep, only to suffer a defeat of catastrophic proportions. Few orcs survive the initial battle.
The Age of Chaos begins with the First War and the death of King Adamant.
The Horde is humiliated by the unexpected rout, and the clans swiftly descend into turmoil. Chaos ensues, with factions blaming each other for the orcs' defeat.
Gul'dan manipulates a ruthless dictator onto the throne of the Warchief. Known as Blackhand the Destroyer, the new Warchief rallies the clans back together again and prepares them for a new assault on Azeroth. The orcs begin raiding activities against outlying human settlements near the Black Morass.
Medivh sends Gul'dan another vision, teasing him with images of the Tomb of Sargeras and the power it contains.
The orcs bring reinforcements surging into Azeroth, decimating towns and villages and plundering everything in their path. The towns of Grand Hamlet and Sunnyglade are destroyed.
Lord Anduin Lothar realizes Medivh's betrayal. He leads a small band of warriors to Medivh’s tower and confronts the wizard. Lothar and Khadgar slay Medivh, but Sargeras' spirit escapes.
While searching the mind of Medivh for the true location of the Tomb of Sargeras, Gul'dan falls in a coma during the death of the Grand Magus, leaving Blackhand and the Shadow Council helpless.
Blackhand the Destroyer is betrayed and slain by Orgrim Doomhammer, a trusted general and close friend. Doomhammer replaces Blackhand as Warchief of the Horde.
Stormwind Keep falls before the full might of the Horde and is sacked. During the battle, King Llane is slain by one of Gul'dan's assassins, the half-orc Garona.
Durotan, chieftain of the Frostwolf clan, and his mate Draka seek an audience with Doomhammer and tell him that Gul'dan seeks to betray the Horde. On their return trip, they are ambushed and slain by orcs loyal to Gul'dan. The assassins leave Durotan's infant son for dead, but the young orc is quickly discovered and saved by a band of humans under the command of Aedelas Blackmoore. The infant is taken as a slave and given the name Thrall.
Spies loyal to Orgrim Doomhammer capture and torture Garona. In agony, she reveals the existence of the Shadow Council and directs them to the location of Gul'dan's warlocks near the ruins of Stormwind Keep.
Doomhammer dispatches his elite wolfriders to the ruins. They slay or execute most of Gul'dan’s warlocks and disperse the remaining members of the Shadow Council. Gul'dan awakens from his coma and pleads for mercy from Doomhammer, offering complete submission. Doomhammer accepts and grants him mercy.
Lord Anduin Lothar concedes that the Kingdom of Azeroth has been lost. He rallies his countrymen and leads them in a desperate retreat across the Great Sea, eventually landing upon the shores of Lordaeron.
The Warlocks resume their experiments with the portal.