Liberation of the orcs from internment camps

The New Horde formed by Thrall and Orgrim Doomhammer with the Warsong and Frostwolf clans liberated the captive orcs in the Alliance of Lordaeron held internment camps before the onset of the Third War.

Lord of the Clans
Thrall's optimism and resilience reignited Orgrim's hopes of restoring the orcs' pride and honor. He made the young shaman his second-in-command. They joined with Grommash Hellscream and his Warsong clan, and they launched a campaign to free the orc prisoners from their internment camps. The lethargy that made the orcs submissive began to fade before the raw energy of Thrall's sense of purpose, and with each toppled camp, the new Horde grew bigger and stronger.

Orgrim Doomhammer fell in battle while liberating one of those camps, and with his last breath, he declared that Thrall should carry on as the new warchief of the Horde. Thrall took up Doomhammer's eponymous weapon, his armor, and his responsibilities, and he sought to dismantle the whole system of internment in one fell swoop.

The Horde marched on Aedelas Blackmoore's fortress, Durnholde Keep, now protected by a group of knights. Thrall offered to parley peacefully. Aedelas responded by executing Taretha Foxton, Thrall's childhood friend. Enraged, Thrall and the Horde stormed the keep, and in a bloody battle, Thrall personally struck down Aedelas.

With Durnholde conquered and the knights defeated, the administration of the internment camps ended instantly. The new Horde had little trouble liberating the rest of the smaller, more isolated camps across Lordaeron. Thrall did not use his numbers to wage wars against Lordaeron itself. Instead, he took his people across the Eastern Kingdoms in search of a place they could call home.

The Alliance splinters
The destruction of the internment camps proved to be the tipping point for many among the Alliance of Lordaeron. The high elves of Quel'Thalas were the first to leave the Alliance. The human nations of Gilneas and Stromgarde soon followed. Gilneas built the Greymane Wall as a way to defend itself without military pacts with the Alliance.

Several kingdoms made it known that they had no intention of letting the Alliance collapse. Stormwind, Lordaeron, Dalaran, Kul Tiras, Ironforge and Gnomeregan all reaffirmed their commitment to unity to whatever trials would come.

By virtue of the Alliance Internment Act, the troops of the Alliance tried to capture the fugitive orcs, offering them to spare their lives if they willingly surrendered. The orcs refused.