Diocletian's tetrarchy did not last for long after his retirement. When Maxentius, who had seized Italy and North Africa, and Constantine, who had succeeded his father to become emperor in 306, had attempted to seize the empire, a conflict developed between the two: each of whom were attempting to rule the Roman Empire. In 312, Constantine defeated Maxentius at Turin and Verona, then at the battle of Milvian Bridge. Constantine chose to share his power with Lucinius, who controlled the east. However, in 316, Constantine chose to attack Lucinius and by 324, Constantine had seized Greece and the Balkans. One man now ruled the entire Roman Empire once again.
Constantine used his power to promote Christianity, the religion that he adopted after he saw a vision of a cross while fighting the battle at the Milvian Bridge. Constantine made Christianity the state religion, and in the process, confiscated temple treasures and built new churches. Constantine took interest in the church, by participating on church councils at Arles in 314 and Nicaea in 325. When he died in 337, he was baptized on his deathbed. Constantine strengthened the security of the empire (especially along the Danube) and he reformed the army by separating the frontier units from the mobile field army. As emperor, he built many structures, including buildings in the first capital, Trier, and in Rome, where he built baths and built the Basilica Nova, whose ruins still dominate the forum. He was nominated the famous Arch of Constantine by the senate, to commemorate his victory over Maxentius. Perhaps Constantine's most impressive achievements was the transformation of the Greek city, Byzantium, into a new capital, Constantinople, in 330.