In addition to nineteen other sector armies, the 18th Sector Army was first activated by 22 BBY, the year that marked the beginning of a civil war—known as the Clone Wars—between the Galactic Republic and the Separatist Rebellion. As one of the larger components in the command structure of the Republic Grand Army, the standard size of the Eighteenth Army was nearly 150,000 clone infantry units, but soon increased dramatically as more clone troopers entered the war. Throughout the duration of the Clone Wars, the Eighteenth was headquartered on the planet Eriadu and charged with the defense of the Greater Seswenna oversector. Like the other sector armies, the overall command of the Eighteenth was shared between a highly experienced Jedi General and a Governor-General who was appointed by the Supreme Chancellor of the Republic. By 19 BBY, the last year of the Clone Wars, the Governor-Generals evolved into a permanent class of regional governors known as the Moffs.
As the Clone Wars progressed, Chancellor Palpatine promoted Wilhuff Tarkin to the rank of Moff, giving him control of the Eighteenth Army and the Seswenna sector. A native-born Human of Eriadu and a former captain in the Republic Navy, Moff Tarkin supplemented the clone units of the Eighteenth with birth-born soldiers of the Outland Regions Security Force in order to better defend the besieged perimeter around his homeworld. By the waning days of the war, Tarkin and his mix of Eighteenth Army and Outland Regions troops were portrayed as Republic heroes through media propaganda due to their extensive action against the Separatists along the Hydian and Rimma.
After the conclusion of the Clone Wars, the Old Republic's twenty sector commands, including the Eighteenth Army, were retained by the newly-risen Galactic Empire. Under the New Order of Emperor Palpatine, Tarkin ascended to the rank of Grand Moff and maintained the Eighteenth Army as a personal power base in his region of the galaxy. Much of the Imperial Era was marked by the Empire's massive military buildup and resulted in a vast army far larger than its Republic predecessor. Thus, the total infantry force of the Eighteenth Army and all other sector units were increased to approximately 1,572,864 troopers, most of whom were of natural birth as clones became marginalized into a minority status.
The Eighteenth Army was first mentioned in the 2009 reference book The Essential Atlas as the Greater Seswenna's Sector Army. It was later mentioned in The Essential Guide to Warfare, a 2012 reference book with extensive information on numerous military themes in Star Wars, including the history of the Sector Army system.
- The Essential Atlas
- The Essential Guide to Warfare