Diocletian wasn't the first emperor to divide the Roman Empire into an eastern and western half. For instance, his predecessor, Carus and sons, Carinus and Numerian(us), had split the Empire, too. However, Diocletian is responsible for the four-part imperial division known as the tetrarchy:
The reign of Diocletian is considered the end of the period of chaos and military anarchy, a time when the armies were picking and assassinating emperors, barbarians were making incursions into Rome, the Persians had become troublesome again, capturing and killing the Emperor Valerian, and emperors were debasing the currency. The tetrarchy was an attempt to create administrative units that could better care for the entire empire.
Diocletian is also one of the emperors involved in the persecution of Christians.
"It was the nineteenth year of Diocletian's reign [AD 303] and the month Dystrus, called March by the Romans, and the festival of the Saviour's Passion was approaching, when an imperial decree was published everywhere, ordering the churches to be razed to the ground and the Scriptures destroyed by fire, and giving notice that those in places of honour would lose their places, and domestic staff, if they continued to profess Christianity, would be deprived of their liberty. Such was the first edict against us. Soon afterwards other decrees arrived in rapid succession, ordering that the presidents of the churches in every place should all be first committed to prison and then coerced by every possible means into offering sacrifice."Tetrarchy
Eusebius, History of the Church (VIII.2)
Diocletian and the Christian Persecution