On this day in 324 AD, the Battle of Chrysopolis was fought in a Roman civil war between the rival emperors Licinius (who controlled the eastern half of the Empire) and Constantine the Great (who controlled the western half).
It was a winner-take-all deal, and had essentially all the troops in the Empire on hand. While ancient sources are notoriously inaccurate on army sizes - the chronicles tell us that Licinius had 170,000 men and Constantine 130,000 - we know that both the Rhein and Danube defensive limes had been stripped by Constantine, and the eastern legions all collected by Licinius. We know this because there was no raiding into an undefended Empire by the barbarian tribes in the west, or by the Persian Empire in the east.
The reason is that both the barbarians and the Persians had been hired as mercenaries by the two rival Emperors, and so rather than raiding were being paid to fight for an Augustus. Like I said, it was a winner-take-all deal, and neither Constantine nor Licinius were of a mind to not bring every sword in their field of influence to the party.
Of course, Constantine won, which is why we call him "Constantine the Great". He went on to establish the christian religion in the Empire, which is why he is know as "Saint Constantine" in the Eastern Orthodox liturgy; indeed, he is counted there as "equal to the Apostles".
But it was a bloody day, with more men at arms gathered in a single place until likely Napoleon's day. The Romans were excellent at organizing, and determined as all get out.