The 300s

300 CE36 entries

301 CE9 entries

• September 3 – The republic of San Marino is established (approximate date).
• November: Diocletian issues his Edict on Maximum Prices, which, rather than halting rampant inflation, causes widespread panic and an increase in inflation. The measure is quickly abandoned.
• King Tiridates III (the Great) proclaims Christianity as the official state religion, making Armenia the first nation to adopt Christianity as its official religion (traditional date). Construction of the original Etchmiadzin Cathedral by Gregory the Illuminator begins.

302 CE10 entries

• Emperor Diocletian persecutes the Manichaeans, accusing them of being a Persian fifth column.
• Iamblichus of Chalcis writes a treatise on magic and the occult.
• Narseh, ruler (Shahanshah) of the Sassanid Empire, dies after a 9-year reign. He is succeeded by his son Hormizd II.

303 CE25 entries

• March 4 – Adrian and Natalia of Nicomedia are martyred.
• September 25 – On a voyage preaching the gospel, Saint Fermin of Pamplona is beheaded in Amiens (Modern day France).
• Etchmiadzin Cathedral is completed by Gregory the Illuminator and Tiridates III, king of Armenia.

304 CE20 entries

• October 25 – Pope Marcellinus dies at Rome after an 8-year reign. The papal throne will remain vacant until 308.
• Cheng-Han earns its independence from Jin dynasty.
• Sixteen Kingdoms: The Xiongnu establishes the Han-Zhao state under Liu Yuan, often seen as the start of the Upheaval of the Five Barbarians.

305 CE23 entries

• May 1 – Emperor Diocletian abdicates and retires to his palace at Salona (modern Split) on the Adriatic coast, after a reign of nearly 21 years, in which the last vestiges of republican government have disappeared.
• Maximinus Daza persecutes the Christians of Egypt, many of whom take refuge in the desert. In time, this refuge leads to the monastic life. In these monasteries, Coptic writing develops, supporting the propagation of Christian texts.
• Catherine makes a public confession of the Christian gospel at a sacrificial feast ordered by Maximinus Daza at Alexandria. A virgin of royal descent, she is tortured on a spiked wheel (later called the "Catherine Wheel") and beheaded; her remains are spirited to Mount Sinai.

306 CE20 entries

• July 25 – Constantius I dies outside Eboracum (modern-day York). Constantine, aged 23 or 24, is declared emperor by his troops. Emperor Galerius grants Constantine the title of Caesar, and elevates Flavius Valerius Severus to emperor in the Western part of the Roman Empire.
• The War of the Eight Princes ends in China.
• Constantine establishes his capital in Augusta Treverorum (Trier). He begins a major expansion of the city, strengthening the walls, expanding the palace complex and building the Imperial Baths.

307 CE12 entries

• January 8 – Emperor Hui of Jin dies after a 16-year reign, in which eight dukes of the imperial family have conducted a civil war (War of the Eight Princes) against each other in a struggle for power. Huai of Jin, age 23, succeeds his father and becomes the third ruler of the Jin Dynasty.
• December: Constantine marries Maximian's daughter Fausta, and is promoted to Augustus by Maximian.
• Galerius sends Valerius Severus with the army of northern Italy, to suppress the rebellion in Rome. However, faced with their former emperor Maximian, the soldiers desert him, and Severus flees to Ravenna. Maximian besieges Severus in Ravenna, who then surrenders. Maxentius makes Severus a hostage, in an attempt to keep Galerius at bay.

308 CE11 entries

• April: In Rome, Emperor Maximian attempts to depose his son Maxentius, but the soldiers in Rome side with Maxentius and force Maximian to flee to the court of Constantine I.
• May 27 or June 26 – Pope Marcellus I succeeds Pope Marcellinus as the 30th pope.
• The Han-Zhao Dynasty is established in northern China, marking the official inception of the long lasting Uprising of the Five Barbarians.

309 CE10 entries

• April 18 – Eusebius succeeds Marcellus I as the 31st pope, but is himself banished on August 17 to Sicily (these events may have also taken place in 310).
• King Hormizd II, ruler of the Sassanid Empire, demands that the king of the Ghassanids pays tribute. After the king refuses, Hormizd invades Ghassanid territory. The Ghassanids seek aid from Maximinus Daza, but before a Roman army can arrive, Hormizd defeats the Ghassanid army and kills their king. A Ghassanid force then ambushes Hormizd's small retinue while the latter is on a hunting trip, and the Sasanian king is mortally wounded. He dies after a 7-year reign.
• Gaius Ceionius Rufius Volusianus, the Praetorian Prefect of Emperor Maxentius, defeats the usurper Domitius Alexander and purges Africa of his supporters.