Following the deaths of Terenas Menethil II and Uther and the corruption of Arthas Menethil, Garithos was left to lead what remained of the Alliance after its fall to the Scourge, which he gathered together to form the "New Alliance". He believed that humans were the only race deserving of life. He hated the Blood Elves most of all, often sending them on suicide misions.
Eventually, he learned that the leader of the Blood Elves, Kael'thas Sunstrider, was working with the Naga and imprissoned Kael and his lieutenants within the dungeons of Dalaran. Shortly after, he sentenced them all to death, but Kael'thas and his lieutenants eventually escaped with the help of one of the Naga leaders, Lady Vashj.
It was not long until Arthas, Kel'Thuzad, and Sylvanas Windrunner returned and resumed their war against humanity. Meanwhile, Illidan Stormrage had severely weakened the Lich King, who was now losing many of his powers. Because of this, Sylvanas and half of the Scourge army had been freed from his slavery and began the Forsaken. Garithos knew he couldn't take on the Scourge and the Dreadlords alone and (with great reluctance) sought help from Sylvanas and her majordomo, Varimathras. This uneasy alliance between the two enemies fought the Scourge, pushing Kel'Thuzad back to Naxxramas.
After Kel'Thuzad's surrender, Balnazzar and Detheroc were all that was left. The second part of the Civil War of the Plaguelands began. While Sylvanas and Varimathras were fighting through Balnazzar's forces, Garithos went on a search for a group of missing Dwarves. The Dwarves were found and Garithos forced them into battle. He and his army entered the fray and Balnazzar was banished back to the Twisting Nether. Garithos then ordered Sylvanas and Varimathras to leave what he believed was his land. Sylvanas, however, had other plans and ordered Varimathras to kill him. Garithos, cought off guard, was murdered on the spot and a team of ghouls arrived to devour the remains.