The Battle of Selsor, also known as the 300 Day Siege, was an engagement during the Galactic Civil War. The Alliance to Restore the Republic controlled an Expansion Region ice planet called Selsor, having over ten thousand recently recruited guerrilla fighters from the planet and other worlds in the sector entrenched there. The Galactic Empire learned of their presence, and worried that the Rebels were planning on making a base on Selsor, attacked immediately. The battle raged for almost a year between the Alliance and Imperial Army, with heavy casualties on both sides, before winter came. At that point, the Stormtrooper Corps deployed the Snow Hawks, an elite snowtrooper unit. They engaged the Rebel forces in what was known as the winter campaign.