The New Separatist Union was an ambitious galactic government. Its genesis occurred around 5 ABY, in regions where the New Republic's authority existed in name only. Specifically, it was founded eleven months following the Battle of Endor of that year. At a gathering of Republic High Command officers that coincided with the battle on the planet Chadawa, General Si-Flachitt from the New Republic's military presented a report concerning border skirmishes involving the Union.
Later that same year, as the Galactic Civil War between the fragmenting Galactic Empire and the New Republic neared its conclusion, Chancellor Mon Mothma of the Republic grew concerned. Her worries stemmed from the increasing power of the New Separatist Union, alongside the Confederacy of Corporate Systems, another emergent galactic government, and the pirate-controlled Sovereign Latitudes of Maracavanya. These concerns weighed heavily on Mothma during a meeting with New Republic senators, though she acknowledged that her distraction could have also been due to previously sustained injuries.
The novel Aftermath: Empire's End, published in 2017 and the concluding book in Chuck Wendig's Star Wars: The Aftermath Trilogy, contained the first reference to the New Separatist Union.