Sargeras began as a Titan, a champion of their concept of an ordered universe. For them he fought Demon's from the Twisting Nether. Over time he became distrustful of the Titan's vision of a good ordered universe, instead believing that chaos was the only constant.
He broke with the other Titans and stormed out of the Pantheon. From there he went on to found the Burning Legion, he also approached the Eredar with promises of power. Three leaders he chose as Lieutenants were Archimonde
. He charged Kil'Jaeden with looking across the whole of exsistence and find the most wicked to be servants of the legion. Archimonde was charged with command of the Legion's armies in battle. Velen however rejected his offer of power for service and became the leader of the Dranei people.
War of the Ancients
Sargeras first became aware of the world of Azeroth
through the Highborne's use of magic at the Well of Eternity. He seduced their leader, Queen Azshara
convincing her that he needed to be brought into the world of Azeroth. Night Elves led by Malfurion Stormrage and Tyrande Whisperwind
were able to defeat the Highborne plan to bring the demon lord to Azeroth but only just. In the process they destroyed the Well which fueled much of Night Elf magic.
The First War
Sargeras was also instrumental in the First War between the orcs and the humans. His avatar came to Azeroth shortly before the first war. He was confronted by the Magical Guardian of Tirisfall Aegwyyn. She defeated Sargeras in combat but his spirit infected her child Medivh.
Aegwynn insisted that only her child, Medivh
, could succeed her as the next guardian of Tirisfal. Corrupted as he was by Sargeras' spirit he communed with the Orcish Warlock Gul'dan and opened a portal from Draenor
, a world nearly destroyed by the orcs under the influence of the Burning Legion.
The Second War
In the second War the warlock Gul'dan
put a premium on finding the Tomb of Sargeras, what he had come to Azeroth for in the first place. He betrayed the Warchief at a critical moment when the Orcs nearly destroyed the alliance in order to go find it taking much of his forces with him.
Gul'Dan found the tomb. He also found that it was not filled merely with magical artifacts, but with insane demons who had been feeding off the corpse of the avatar of Sargeras for decades. The demons destroyed Gul'dan.
The Third War
The third war was carried out almost entirely without Sargeras' command. Archimonde, Mannoroth
and Kil'jaden had taken command of the legion in the absence of a corporeal Sargeras. Near the end of Warcraft III
it however becomes clear that Medivh in death has been freed of Sargeras' dark influence as he is revealed to be the Prophet guiding Thrall