The Secession Wars was a conflict between the United Nations Space Alliance and the Settlement Defense Front. This war took place 30 years prior to the events of Call of Duty: Infinite Warfareand is named as such due to the SDF's secession from the UNSA. The war intially started out as a mere conflict between the earth-led colonial government of Mars and group of militarist radicals (the SDF) but eventually escalated into a full-scale war and spilled over, spreading out into the rest of the Solar System.
The war was ultimately fought to a stalemate, resulting in the Charon Accords which established the Settlement Defense Front as an independent, totalitarian state separate from the United Nations Space Alliance with Mars being designated as its capital.
The Charon Accords peace, however, was a fragile one with many territories in the Solar System remaining contested between the two states and occasional border skirmishes occurring even in peacetime, before an SDF attack on Earth finally broke the treaty, leading to the outbreak of another war .