The Battle of Adrianople, 323

 It seems Constantine *accidently* strayed into Licinius' territory whilst giving chase to some Sarmatians who had crossed the Danube into imperial territory, I suspect Constantine fully intended to do this and provoke Licinius.

 I know a little about this battle both not much on details except that both sides were apparently huge, Licinius may have had as many as 150,000 men + Constantine having slightly less but easily over 120,000. The difference being Constantines army had many battle-hardened veterans within it and despite Licinius putting up brave resistance he ordered his army to withdraw after 30,000 of so of his men were dead on the field.

 After stopping at Byzantium Licinius confronted Constantine again Chrysopolis where he was again defeated.

 I've found it hard to come across more than just a brief analysis of the battle similar to what i've given, as far as tactics and the length of the battle etc go ive been unable to uncover anything. Anyway this'll do as a broad outline of the battle and events.

This is what Gibbon has to say about it in his "Decline and Fall"

In this exalted state of glory, it was impossible that Constantine should any longer endure a partner in the empire. Confiding in the superiority of his genius and military power, he determined, without any previous injury, to exert them for the destruction of Licinius, whose advanced age and unpopular vices seemed to offer a very easy conquest. But the old emperor, awakened by the approaching danger, deceived the expectations of his friends, as well as of his enemies. Calling forth that spirit and those abilities by which he had deserved the friendship of Galerius and the Imperial purple, he prepared himself for the contest, collected the forces of the East, and soon filled the plains of Hadrianople with his troops, and the Straits of the Hellespont with his fleet. The army consisted of one hundred and fifty thousand foot, and fifteen thousand horse; and as the cavalry was drawn, for the most part, from Phrygia and Cappadocia, we may conceive a more favorable opinion of the beauty of the horses, than of the courage and dexterity of their riders. The fleet was composed of three hundred and fifty galleys of three ranks of oars. A hundred and thirty of these were furnished by Egypt and the adjacent coast of Africa. A hundred and ten sailed from the ports of Phoenicia and the Isle of Cyprus; and the maritime countries of Bithynia, Ionia, and Caria, were likewise obliged to provide a hundred and ten galleys. The troops of Constantine were ordered to a rendezvous at Thessalonica; they amounted to above a hundred and twenty thousand horse and foot. Their emperor was satisfied with their martial appearance, and his army contained more soldiers, though fewer men, than that of his eastern competitor. The legions of Constantine were levied in the warlike provinces of Europe; action had confirmed their discipline, victory had elevated their hopes, and there were among them a great number of veterans, who, after seventeen glorious campaigns under the same leader, prepared themselves to deserve an honorable dismission by a last effort of their valor. But the naval preparations of Constantine were in every respect much inferior to those of Licinius. The maritime cities of Greece sent their respective quotas of men and ships to the celebrated harbor of Pir�us, and their united forces consisted of no more than two hundred small vessels -- a very feeble armament, if it is compared with those formidable fleets which were equipped and maintained by the republic of Athens during the Peloponnesian war. Since Italy was no longer the seat of government, the naval establishments of Misenum and Ravenna had been gradually neglected; and as the shipping and mariners of the empire were supported by commerce rather than by war, it was natural that they should the most abound in the industrious provinces of Egypt and Asia. It is only surprising that the eastern emperor, who possessed so great a superiority at sea, should have neglected the opportunity of carrying an offensive war into the centre of his rival's dominions.

Instead of embracing such an active resolution, which might have changed the whole face of the war, the prudent Licinius expected the approach of his rival in a camp near Hadrianople, which he had fortified with an anxious care, that betrayed his apprehension of the event. Constantine directed his march from Thessalonica towards that part of Thrace, till he found himself stopped by the broad and rapid stream of the Hebrus, and discovered the numerous army of Licinius, which filled the steep ascent of the hill, from the river to the city of Hadrianople. Many days were spent in doubtful and distant skirmishes; but at length the obstacles of the passage and of the attack were removed by the intrepid conduct of Constantine. In this place we might relate a wonderful exploit of Constantine, which, though it can scarcely be paralleled either in poetry or romance, is celebrated, not by a venal orator devoted to his fortune, but by an historian, the partial enemy of his fame. We are assured that the valiant emperor threw himself into the River Hebrus, accompanied only by twelve horsemen, and that by the effort or terror of his invincible arm, he broke, slaughtered, and put to flight a host of a hundred and fifty thousand men. The credulity of Zosimus prevailed so strongly over his passion, that among the events of the memorable battle of Hadrianople, he seems to have selected and embellished, not the most important, but the most marvellous. The valor and danger of Constantine are attested by a slight wound which he received in the thigh; but it may be discovered even from an imperfect narration, and perhaps a corrupted text, that the victory was obtained no less by the conduct of the general than by the courage of the hero; that a body of five thousand archers marched round to occupy a thick wood in the rear of the enemy, whose attention was diverted by the construction of a bridge, and that Licinius, perplexed by so many artful evolutions, was reluctantly drawn from his advantageous post to combat on equal ground on the plain. The contest was no longer equal. His confused multitude of new levies was easily vanquished by the experienced veterans of the West. Thirty-four thousand men are reported to have been slain. The fortified camp of Licinius was taken by assault the evening of the battle; the greater part of the fugitives, who had retired to the mountains, surrendered themselves the next day to the discretion of the conqueror; and his rival, who could no longer keep the field, confined himself within the walls of Byzantium.

Good information Decebal.