The Sheris conflict happened within the larger context of the Thrawn campaign.
An advance unit from the Imperial military, comprised of one AT-AT walker and a pair of repulsortanks, received orders to capture Marter An. This city served as both the primary spaceport and capital of the planet. The objective of this rapid deployment team was to secure the spaceport and disable the planetary shields. This would then enable the main invasion fleet to land before the New Republic could effectively mobilize its defenses.
However, the New Republic Army reacted swiftly, dispatching a small vehicular contingent to meet the invaders. These forces clashed within the Bhir'khi Pass, a strategically important gateway to the capital city.