Passage: The underlying causes of the battle were the rivalries inherent in Diocletian's Tetrarchy.  After Diocletian stepped down on 1 May 305, his successors began to struggle for control of the Roman Empire almost immediately. Although Constantine was the son of the Western Emperor Constantius, the Tetrarchic ideology did not necessarily provide for hereditary succession. When Constantius died on 25 July 306, his father's troops proclaimed Constantine as Augustus in Eboracum (York). In Rome, the favorite was Maxentius, the son of Constantius' imperial colleague Maximian, who seized the title of emperor on 28 October 306. But whereas Constantine's claim was recognized by Galerius, ruler of the Eastern provinces and the senior emperor in the Empire, Maxentius was treated as a usurper. Galerius, however, recognized Constantine as holding only the lesser imperial rank of Caesar. Galerius ordered his co-Augustus, Severus, to put Maxentius down in early 307. Once Severus arrived in Italy, however, his army defected to Maxentius. Severus was captured, imprisoned, and executed. Galerius himself marched on Rome in the autumn, but failed to take the city. Constantine avoided conflict with both Maxentius and the Eastern emperors for most of this period.By 312, however, Constantine and Maxentius were engaged in open hostility with one another, although they were brothers-in‑law through Constantine's marriage to Fausta, sister of Maxentius. In the spring of 312, Constantine gathered an army of 40,000 soldiers and decided to oust Maxentius himself. He easily overran northern Italy, winning two major battles: the first near Turin, the second at Verona, where the praetorian prefect Ruricius Pompeianus, Maxentius' most senior general, was killed.

Question: what was the main reason for fighting?
rivalries inherent