The Battle of Sheris took place during the Thrawn campaign.
A small Imperial vanguard force, consisting of an AT-AT and two repulsortanks, was sent to seize Marter An, the capital city and major spaceport of the planet. This strike team was supposed to take control of the spaceport and destroy the planetary shields in orer to allow the landing of the main invasion force before the organization of the New Republic defense forces.
But the New Republic Army was faster and sent a small group of vehicles to intercept them. The New Republic and Imperial forces fought in the Bhir'khi Pass, a strategic access point to the capital.