It is unknown where the Sentients came from, how they came into contact with the Orokin or what began the war. At least one battle took place around a "dark and blinding" blue alien star. What is known, however, is that they had the ability to subvert any technology or advanced weapon that was used against them. Since the Orokin were an eminently technological faction, this forced them to use primitive weapons to fight them, "zero-tech" equipment like percussion rifles, "Splinter ships" and bladed melee weapons, which ultimately proved ineffective.
Desperate for a countermeasure in their losing war, the Orokin engineered the "Technocyte Virus", creating the first Infested (supposedly out of dead bodies or perhaps out of their own populace) as semi-technological bioweapons. However the plague backfired, affecting many of the Orokin's own ships and settlements before they could take refuge into the Void. The Infested proved impossible to control, ultimately making the situation worse for the Orokin.
It is not known for certain if the Infested were effective against the Sentients, but they were most likely not, since the Orokin resorted to their most desperate option: the Tenno. Originally people who came into direct contact with the environment of the Void and survived, becoming afflicted in an unspecified manner while also possessing powerful abilities (which not even the Orokin fully understood), they were trained and equipped with Warframes, specially designed armor suits created to contain and channel their power. Thus the Tenno, regarded as monsters, demons, ghosts or mere propaganda myths by many people of the Orokin Empire, were made into the perfect warriors to fight the Sentients, since they were not reliant on advanced technologies, but rather on their own Void-derived power.
The final fate of the Sentients is unknown, but the Tenno returned victorious to their Orokin masters and slaughtered them all.