After the Battle of Yavin, the rebels fled from the Imperial starfleet, seeking a new main base of operations. They settled on the frozen Outer Rim world of Hoth, where they enlarged caverns to form Echo Base, defended by a powerful ion cannon and squads of airspeeders converted to operate in frigid conditions. Unfortunately, the base was barely established when an Imperial probe droid discovered it. General Rieekan ordered an immediate evacuation, but before Echo Base could be abandoned an Imperial task force arrived, landing AT-AT walkers and ground troops. Though key personnel escaped, the Battle of Hoth was a costly defeat for the Alliance.