Whilst undoubtably a great Emperor, Diocletian (284/305), has a couple of historical black marks against his name.
1) The Tetrarchy (a system 2 senior emperors and 2 junior emperors). Diocletian saw the problem of one man ruling such a vast empire and also observed the chaos created by usurping generals in the mid third century. The system was supposed to provide stable government with senior emperors bringing on juniors who in turn would have Caesars to support them. It failed however as soon as Diocletian resigned and the renewed civil war was only finished when Constantine eliminated Licinius in 324 and became sole emperor (though he in turn, left the empire to his three sons and two nephews and so created a another bout of civil wars after his death).
2) His persecution of the christians in 303. Diocletian - prompted by his anti Christian junior Caesar Galerius, imposed strict restrictions on Christians, banishing them from civil service and the army, making them hand over their scriptures and, most tellingly, perform a pagan sacrifice. Many Christians refused and were killed in a variety of awful ways. It is here that St George comes in. A top general in the army, but a Christian, George refused to recant his Christianity and so was martyred by having his head chopped off after torture. Hence St George.
This story is probably a bit more likely than some nonsense about a knight slaying a dragon and rescuing a princess. The persecution did take place and many martyrs were created. To be honest, this is a better, more interesting story than the dragon rubbish. Why is it we were never taught this at school? It combines classical history, the early birth of christianity and - yes - fables.
Anyway, whatever, Happy St George's Day.