Hoth is the sixth planet in the system of the same name. After the Battle of Yavin, Luke Skywalker proposed Hoth as the Rebel Alliance's new main base of operations after they evacuated their base on Yavin 4. The Alliance leaders agreed and Echo Base became the new Rebel base. To prevent the risk of total destruction that the Alliance faced during the Battle of Yavin, Luke also set up an evacuation protocol that stated that the base was to be evacuated as soon as it was discovered by the Empire.
Three years after the Battle of Yavin, Vader learned what sector the base was in and sent Imperial Probe Driods to seach for it. When one of the Droids landed on Hoth, Luke spoted it but his ambush by a Wampa ensured that the Probe remained undiscovered until it was too late because by the time Han Solo discovered and destroyed it the droid had already found the base and the Rebels suspected that the Empire knew where they were and initiated Luke's evacuation protocol.
The Rebels' fears proved correct: Before it's destruction, the driod transmitted it's finding to Death Squadron, the personal armada of Darth Vader. Captain Firmus Piett realized that the probe had found the rebel base. Death Squadron's commander, Admiral Kendal Ozzel, dismissed the report as a smuggler's settlement but Darth Vader confirmed otherwise and ordered Ozzel to set course for Hoth. Vader origianally planned for Death Squadron to arrive at the edge of the Hoth System and use the system's asteroids to conceal the armada's arrival. Upon arriving, Death Squardron would initaiate orbital bomardment to destroy the base and flush out the Rebels where they would be captured by General Maximillon Veers, the commander of Death Squadron's ground forces. However, Ozzel thought it was wiser to exit light speed as close to the planet as possible to catch the Rebels by surprise and had the fleet do precisely that. This proved to be a mistake because he brought the fleet too close within the base's sensors which alerted the Rebels to the armada's arrival causing them to raise a energy sheild powerful enough to protect the base from orbital bombardment. Furious, Vader executed Ozzel for his blunder and promoted the more competent Piett to Admiral and made him Commander of Death Squadron.
Veers and his troopers were then deployed for a surface attack. Led by Luke Skywalker, Rouge Squadron flew out in Speeders to hold Veers's forces off while the rest of the evacuation took place and even managed to down a few of the AT-ATs. However, because of the sheer might of Death Squadron, Veers was able to take out Echo Base's sheild generator allowing Vader and the 501st Legion to overrun the base. The Battle of Hoth was a major Imperial victory that turned the tide in the favor of the Empire for the next six months however because of Luke's evacuation protocal, Ozzel's blunder, and Echo Base's Ion Cannon seventeen transports worth of soilders, personal, and equipment was able to escape the base's destruction.