The Siege of Segusio (Latin: Segusium) or Siege of Susa was the first clash of the civil war between the Roman emperors Constantine the Great (r. 306–337) and Maxentius (r. 306–312) in the spring of 312. In that year, Maxentius had declared war on Constantine, claiming to intend to avenge the death of his father Maximian (r. 285–308, 310), who had committed suicide after being defeated by him. Constantine would respond with an invasion of northern Italy.