Sector Army/Legends

A Sector Army was a type of military formation used by the Grand Army of the Republic and, later, the Imperial Army and the Imperial Navy, although the role and composition of these units evolved markedly between the Clone Wars and the Galactic Civil War.

Sector Armies in the Clone Wars
In the Clone Wars, Sector Armies were the main subordinate component units of the ten Systems Armies that formed the military backbone of the Grand Army of the Republic. There were two Sector Armies in each Systems Army, making a total of twenty Sector Armies; each of them was commanded by a Senior Jedi General, and had a nominal combat strength of 147,456 troopers, divided into four Corps (although it should also be noted that many of the Legions which made up the Corps also included "Corps" within their unit names).

The 2nd Sector Army, commanded by Jedi Master Ry-Gaul, was part of Systems Army Alpha, while the 4th Sector Army, known unofficially as the "Outer Rim Sector Army", was the main GAR force in the Outland regions. We know that the 2nd Sector Army numbered among its component units the 327th Star Corps, led by Commander Bly and Jedi Master Aayla Secura, and the 4th Sector Army was the parent of the 21st Nova Corps, Ki-Adi-Mundi's Galactic Marines

Sector Armies under the Empire
Under the New Order, the number of Sector Armies rose dramatically; each of the Moffs, the Governors who Palpatine dispatched to oversee the thousand Sectors which the Empire had inherited from the Old Republic, bore the title of Surface Marshal of a Sector Army. And each of these Sector Armies could serve in turn as the high command for an Assault Fleet capable of deploying four full surface Armies against any resistance or hostility in the sector - 774,576 combat troops in a total muster of 1,180,379 personnel - to say nothing of a number of autonomous Corps HQs for garrison duty.

List of Sector Armies under the Clone Wars and Imperial Period

 * 1st Sector Army - Coruscant/Imperial Center Oversector
 * 2nd Sector Army - Corellian Oversector
 * 3rd Sector Army - Sector 3
 * 4th Sector Army - Sector 4
 * 5th Sector Army - Sector 5
 * 6th Sector Army - Sector 6
 * 7th Sector Army - Immalia Oversector
 * 8th Sector Army - Ord Mantell Oversector
 * 9th Sector Army - Relgim Oversector
 * 10th Sector Army - Quelii Oversector
 * 11th Sector Army - Gordian Reach Oversector
 * 12th Sector Army - Maldrood Oversector
 * 13th Sector Army - Trans-Nebular Oversector
 * 14th Sector Army - Tolonda Oversector
 * 15th Sector Army - Sarin Oversector
 * 16th Sector Army - Pallis Oversector
 * 17th Sector Army - Dustig Oversector
 * 18th Sector Army - Greater Seswenna
 * 19th Sector Army - Javin Oversector
 * 20th Sector Army - Tapani Oversector

Behind the scenes
Sector Armies were introduced as elements of the Imperial Army in the Imperial Sourcebook, and retrospectively given their Clone Wars back-story only in the Guide to the Grand Army of the Republic. Obi-Wan Kenobi's Third Army can probably be identified as another GAR Sector Army (and in turn, the 3rd and 4th Sector Armies probably combined to form Systems Army Beta), but there seems to be no specific confirmation of this. In the Imperial Sourcebook, the most obvious reading of the text is that a Sector Army is divided for administrative purposes into two Systems Armies and represented in combat by four front-line Armies and an associated Navy Assault Fleet; but this enumeration cannot represent the entire order of battle of a standard Sector Army, as it omits the Corps HQs and combat battalions of planetary garrison, each of which was designed as the basis to build up a new combat Corps on a world when required. It is very possible that the number of Systems Armies, surface Armies, front-line Corps and garrison details in a Sector Army should be thought of as varying markedly from sector to sector.