By 312, the Roman Empire had reached a turning point after centuries of military dictatorship powered by the wheels of bureaucracy.
Maxtenius Victorious at Milvian Bridge Since the domination of Octavian over Julius Caesar's assassins, the Senate had been largely a stamp for the emperor to pass his decrees. Many men pursued this utmost position, and civil wars erupted often when capable generals overtook weak emperors. The empire itself became unwieldy, and Diocletian divided Rome into western and eastern parts with co-rulers in each. By the early fourth century, further divisions and murky agreements had created a Tetrarchy where four men controlled the empire as Caesars and Augusti.
In 306, Augustus Constantius Chlorus died, and his son Constantine was proclaimed by his soldiers on the frontier of Britannia that he would be the new emperor. Currently controlling Rome, however, was Maxentius, who had taken the title of Augustus by force after defeating Severus, the legal appointee by the eastern Augustus, Galerius. Licinius, another would-be emperor, had been proclaimed emperor by a conference of the leading political figures of Rome. By 312, Constantine was already moving on Rome to defeat the usurper Maxentius and making plans for alliance with Licinius.
Constantine organized the execution of Maxentius's father, Maximinian, and marched with an army of some 40,000, racing over northern Italy and defeating armies more than twice his size, even killing Maxentius's highest general, Ruricius Pompeianus, at Verona. Maxentius had already held Rome successfully through two sieges, but he decided to deal with the upstart from the north himself, setting up an army on the far side of the Milvian Bridge over the Tiber River. On the evening of the 27th, Constantine's forces prepared for battle the next day, and a vision came upon them. Looking into the setting sun, they saw a cross made of light and words in Greek reading, "In this sign, conquer". A dream that night explained that the sign was from a sect of worshippers of the Hebrew god, practically the only one without a temple in Rome where Maxentius had already made substantial sacrifices toward victory in the battle.
As the morning dawned, Constantine prepared his men to mark the sign on their shields, but he was unnerved by the use of Greek letters chi and rho spelling the first sounds of "Christ" when the chi could have easily been his own "Constantine". Hubris came over him, and he edited the sign for his soldiers from the "P" into an "O" for the omicron that would spell his own second letter. The move would prove disastrous, as the rounded shape formed a handy target at the top of the Roman shields where they would be knocked into the faces of their bearers, distracting them while missiles or blows from swords followed. Despite losing the opening cavalry skirmish, Maxentius's army won the day and pressed Constantine's army into breaking. Constantine himself was killed while trying to rally his retreating soldiers.
Maxentius returned victoriously to Rome. Constantine's onetime ally, Licinius, had overseen affairs in the east along with Maximinius Daia but now sought to support Maxentius. Encouraged by Maxentius's victory, Maximinius attempted to overthrow Licinius with an invasion of Byzantium, but Licinius defeated him at Tzirallum and pursued him to utter defeat and suicide at Tarsus. The remainder of Licinius' reign was spent holding off Sassanid attack, while Maxentius went about legitimizing himself and working to stitch the eastern empire back to dependency on Rome as he lent Licinius great masses of wealth to aid in defense.
A century later, the Roman Empire would fall as German barbarians stormed across the Alps and repeatedly sacked and finally conquered the Eternal City in 476. Without a particular seat of strength in the east, the rest of the empire shattered into bases of power in Egypt, the Bosporus, and Syria. The end of Roman authority finally meant an end to centuries-long persecution of the Christian sects, whose monotheism was grown out of Jewish doctrine. With a plethora of Egyptian, Babylonian, Greek, Roman, and now German gods in veneration, monotheism would serve as a minority in Europe. Norse gods would come to dominate during the Viking Age, but the cohesion of Allah in the Arabic Islam would eventually sweep across Europe, Africa, and well into Asia, carried even further by converted Mongol conquerors a millennium later.