304

Year 304 was a leap year starting on Saturday of the Julian calendar. It was known in the Roman Empire as the Year of the Consulship of Diocletian and Maximian. The denomination 304 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

Events

Caesar Galerius, perhaps accompanied by Emperor Diocletian, wins his fourth and final victory over the Carpi. Many of the surviving Carpi and Bastarnae are resettled in the Roman Empire, where they are split up. The Bastarnae are not attested after this time, and the Carpi are attested only once more in the 310s.
Diocletian, while inspecting the Danube border, becomes seriously ill.
Caesar Constantius I besieges a Germanic raiding force on an island in the Rhine and forces their surrender.
Asia
Sixteen Kingdoms: The Xiongnu establishes the Han-Zhao state under Liu Yuan, often seen as the start of the Upheaval of the Five Barbarians.
Cheng-Han earns its independence from Jin dynasty.
Biryu becomes king of the Korean kingdom of Baekje.
By topic

October 25 - Pope Marcellinus dies at Rome after an 8-year reign. The papal throne will remain vacant until 308.
Deaths

Note: Not all particulars of the Christian martyrs are supported by reliable historical evidence.