Diocletian united and stabilized the Empire, then divided it into 2 pieces with 2 emperors each. Shortly after he and his co-emperor retired in 305, the system broke down. Constantine and Maxentius succeeded their fathers in the West, and Licinius and Maximinus became emperors in the East. Very quickly, they began battling each other for supremacy, mostly with the two in the East fighting each other and the two in the West fighting each other.
The sign in the sky
Eusebius recounts the previous scene as follows, in Life of Constantine:
Accordingly he called on him with earnest prayer and supplications that he would reveal to him who he was, and stretch forth his right hand to help him in his present difficulties. And while he was thus praying with fervent entreaty, a most marvelous sign appeared to him from heaven, the account of which it might have been hard to believe had it been related by any other person. But since the victorious emperor himself long afterwards declared it to me, when I was honored with his acquaintance and society, and confirmed his statement by an oath, who could hesitate to accredit the relation, especially since the testimony of after-time has established its truth? He said that about noon, when the day was already beginning to decline, he saw with his own eyes the trophy of a cross of light in the heavens, above the sun, and bearing the inscription, “By this sign, conquer.” At this sight he himself was struck with amazement, and his whole army also, which followed him on this expedition, and witnessed the miracle.
The battle of the Milvian Bridge
Again, from the Life of Constantine:
So at this time Maxentius, and the soldiers and guards with him, “went down into the depths like stone,” when, in his flight before the divinely-aided forces of Constantine, he essayed to cross the river which lay in his way, over which, making a strong bridge of boats, he had framed an engine of destruction, really against himself, but in the hope of ca-snaring thereby him who was beloved by God. For his God stood by the one to protect him, while the other, godless, proved to be the miserable contriver of these secret devices to his own ruin. So that one might well say, “He who digs a hole and scoops it out falls into the pit he has made. The trouble he causes recoils on himself; his violence comes down on his own head. ” Thus, in the present instance, under divine direction, the machine erected on the bridge, with the ambuscade concealed therein, giving way unexpectedly before the appointed time, the bridge began to sink, and the boats with the men in them went bodily to the bottom.
The Edict of Milan
Opening text of the Edict of Milan
When I, Constantine Augustus, as well as I, Licinius Augustus, fortunately met near Milan, and were considering everything that pertained to the public welfare and security, we thought, among other things which we saw would be for the good of many, those regulations pertaining to the reverence of the Divinity ought certainly to be made first, so that we might grant to the Christians and others full authority to observe that religion which each preferred; whence any Divinity whatsoever in the seat of the heavens may be propitious and kindly disposed to us and all who are placed under our rule. And thus by this wholesome counsel and most upright provision we thought to arrange that no one whatsoever should be denied the opportunity to give his heart to the observance of the Christian religion, of that religion which he should think best for himself, so that the Supreme Deity, to whose worship we freely yield our hearts, may show in all things His usual favor and benevolence. Therefore, your Worship should know that it has pleased us to remove all conditions whatsoever, which were in the rescripts formerly given to you officially, concerning the Christians and now any one of these who wishes to observe Christian religion may do so freely and openly, without molestation.
Constantine vs. Licinius
Relationships between the two emperors quickly deteriorated, of course…