The Eighteenth Army, alternatively called the Night Hammer Command or the Greater Seswenna's Sector Army, represented one of the twenty sector armies that formed the Grand Army of the Republic during the Clone Wars. This conflict was a civil war fought between the Galactic Republic and the Confederacy of Independent Systems. As the galaxy-wide war neared its end, the 18th Sector Army came under the leadership of Moff Wilhuff Tarkin, who was also the Governor-General of Sector 18. Following the Republic's transition into the Galactic Empire, the army continued its military service.
Alongside nineteen other sector armies, the 18th Sector Army was initially activated around 22 BBY. This year marked the beginning of the Clone Wars, a civil war between the Galactic Republic and the Separatist Rebellion. Serving as a significant element within the Republic Grand Army's command structure, the Eighteenth Army typically consisted of approximately 150,000 clone infantry units, a number that increased substantially as more clone troopers joined the fight. Throughout the Clone Wars, the Eighteenth was based on the planet Eriadu and tasked with defending the Greater Seswenna oversector. Like other sector armies, the Eighteenth's command was shared between a seasoned Jedi General and a Governor-General appointed by the Republic's Supreme Chancellor. By 19 BBY, the Clone Wars' final year, the Governor-Generals transformed into a permanent group of regional governors known as the Moffs.
As the Clone Wars progressed, Chancellor Palpatine elevated Wilhuff Tarkin to the rank of Moff, granting him control over both the Eighteenth Army and the Seswenna sector. Tarkin, a Human born on Eriadu and a former captain in the Republic Navy, augmented the Eighteenth's clone forces with native-born soldiers from the Outland Regions Security Force. This was done to strengthen the defenses around his homeworld, which was under siege. Towards the end of the war, Tarkin and his combined forces of the Eighteenth Army and Outland Regions troops were celebrated as Republic heroes in media propaganda due to their extensive campaigns against the Separatists along the Hydian and Rimma trade routes.
Following the Clone Wars' end, the Old Republic's twenty sector commands, including the Eighteenth Army, were maintained by the newly-risen Galactic Empire. Under Emperor Palpatine's New Order, Tarkin rose to the rank of Grand Moff and used the Eighteenth Army as a source of personal power within his region of the galaxy. The Imperial Era was largely defined by the Empire's massive military buildup, resulting in a vast army that dwarfed its Republic predecessor. Consequently, the total infantry strength of the Eighteenth Army and all other sector units was increased to roughly 1,572,864 troopers, with most being naturally born as clones were relegated to a minority role.
The Greater Seswenna's Sector Army, later identified as the Eighteenth Army, was initially mentioned in the 2009 reference book called The Essential Atlas. Subsequently, it appeared in The Essential Guide to Warfare, a 2012 reference book that provided detailed information on various military aspects within Star Wars, including the history of the Sector Army system.