BATTLE OF HOTH
One of the worst battlefield defeats suffered by the Rebel Alliance at the hands of the Empire, this loss would have been worse if an Imperial commander hadn't made a tactical error. After the Rebel victory at the Battle of Yavin, the Alliance relocated its command base centre, Echo Base, to Hoth. Despite its terrible climate, Hoth seemed like a good hiding place, but Echo Base hadn't been completed when an Imperial probe droid discovered the Rebels.
If Admiral Ozzel hadn't brought the Imperial Fleet out of hyperspace too close to the Hoth system, thus alerting the Rebels and allowing many to evacuate, the Alliance's huge losses would have been even greater. Alliance shields were activated to protect the base from space bombardment for a short time, letting the Rebels evacuate staff and material. Imperial Star Destroyers quickly moved into position and ground forces unleashed fearsome AT-AT walkers and legions of snowtroopers.
The Alliance has no choice but to engage in combat with the Imperial walkers using modified airspeeders, later dubbed "Snowspeeders". Heavy losses ensued the battle. Nevertheless, most of the Rebel command personnel and a large amount of equipment made it off of Hoth. The successful evacuation led the Rebellion in a final do-or-die situation, an all-out attack at the Battle of Endor a year later.