During the Nova Terra Interregnum, the whole of Segmentum Pacificus was claimed by the Ur-Council of Nova Terra. Later thousands of worlds in Segmentum Pacificus were claimed by the Imperium in the Macharian Crusade, but many were subsequently lost in the Macharian Heresy.
In 999.M41, whole swaths of Segmentum Pacificus would suffer from mass uprisings by disgruntled citizens collectively known as the Night of a Thousand Rebellions. Many outlaying planets as well as key worlds such as Darkhold, Minisotira, and Enceladus are swept up in the chaos. Even the Lions Defiant Chapter of Space Marines find their Homeworld the victim of insurgent activity. Since the uprisings began, communication with much of Segmentum Pacificus has been lost.