300s (decade)

The 300s decade ran from January 1, 300, to December 31, 309.

• Emperor Diocletian begins construction of a palace that will become the city of Split (approximate date). Diocletian, who plans on abdicating, intends to use this palace as his place of retirement.
• Caesar Constantius I wins a victory over the Franks (approximate date).

• The lion becomes extinct from Armenia (approximate date).
• The Yayoi period ends in Ancient Japan (approximate date).
• Wootz steel is developed in India (approximate date).
• The Kama Sutra, an Indian handbook on the art of sexual love, is probably produced around this time by the sage Vatsyayana.
• Micheon becomes ruler of the Korean kingdom of Goguryeo.

• The elephant becomes extinct in North Africa (approximate date).
• The Atlas wild ass becomes extinct (approximate date).

• The Formative/Preclassic period in Mesoamerica comes to an end approximately around this year.
• The Mayan civilization reaches its most prolific period, the classic period, in what is now Guatemala, Belize and parts of southern Mexico adjacent to the former two. During most of this period, Tikal dominates the Mayan world.

• The magnetic compass for navigation is invented in China (approximate date).
• The Panchatantra, a Sanskrit collection of fables and fairy tales, is written in India.
• The Tetrarchs are probably made in Egypt. After 330 they are moved to Constantinople and in 1204 they are installed at the corner of the facade of the St Mark's Basilica, Venice (approximate date).
• Diocletian's Palace, Split, Croatia, is built. Its model is nowadays kept at the Museo della Civilta Romana, Rome.

• Peter of Alexandria becomes Patriarch of Alexandria.
• Possible date of the Codex Vaticanus Graecus 1209 and Codex Sinaiticus, manuscripts of the Bible written in Greek.
• Approximate date of the Synod of Elvira in Elvira, Spain, which prohibits interaction with Jews, pagans, and heretics.

• Caesar Galerius begins a major war against the Carpi and Bastarnae and wins the first of several victories.
• September: Emperor Diocletian issues a reform that revalues the Roman currency.
• 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.

• September 3 – The republic of San Marino is established (approximate date).

• February 3 – May 30 – Sima Lun briefly usurps the Jin Dynasty.

• Emperor Diocletian persecutes the Manichaeans, accusing them of being a Persian fifth column.
• Caesar Galerius wins his second victory over the Carpi.
• An invasion of Gaul by the Alemannic Lingones almost traps Caesar Constantius I between the enemy and the walls of a town. Constantius himself is carried onto the wall via a crane. However, within the same day, Constantius sallies forth from the walls and defeats the enemy in a major battle.

• Narseh, ruler (Shahanshah) of the Sassanid Empire, dies after a 9-year reign. He is succeeded by his son Hormizd II.

• Iamblichus of Chalcis writes a treatise on magic and the occult.

• Gregory the Illuminator is consecrated as Patriarch of Armenia by Leontius of Caesarea.

• Great Persecution: Emperor Diocletian launches the last and largest major persecution of Christians in the Empire. Caesar Galerius and Hierocles are said to have been the instigators. In a series of four edicts published from February 23, 303, to 304, the Christians are forbidden to worship in groups, are made to perform sacrifices, and must surrender sacred texts. Churches are destroyed, and the clergy are arrested en masse. The persecution lasts in some parts of the empire until 313, and thousands of Christians are killed. Those put to death include Agnes of Rome, a 12-year-old Christian girl who has refused marriage and consecrated her virginity to God. Hailed as a martyr, she will be honored as the patron saint of chastity, gardeners, rape victims and virgins.
• September 25 – On a voyage preaching the gospel, Saint Fermin of Pamplona is beheaded in Amiens (Modern day France).
• November 20 – The Augusti Diocletian and Maximian reunite in Rome to celebrate the 20th anniversary of Diocletian's accession, which is now treated as a joint anniversary for both emperors. A series of columns in the Roman Forum and a triumphal arch are dedicated to the emperors. The two emperors also agree on a plan of abdication.
• Galerius wins his third victory over the Carpi and is perhaps joined on campaign by Diocletian. The Arch of Galerius is dedicated in Thessaloniki.
• Caesar Constantius I wins a victory over Germanic invaders in the battle of Vindonissa.

• Etchmiadzin Cathedral is completed by Gregory the Illuminator and Tiridates III, king of Armenia.
• January 6 – Baptism of Tiridates III of Armenia.

• In Mexico, the civilization of Teotihuacan flourishes.

• March 4 – Adrian and Natalia of Nicomedia are martyred.

• 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.

• 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.

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

• 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.
• Maximian retires from office, and leaves for Campania or Lucania (southern Italy), to live a life of ease in his luxury villas.
• Constantius I and Galerius are declared Augusti; Flavius Valerius Severus and Maximinus Daza are appointed Caesars.
• Constantius requests leave for his son Constantine I to join him in the west, who has been living at the courts of Diocletian and Galerius as a hostage. Galerius allows Constantine to return.
• Summer – Constantine joins his father at Bononia (Boulogne) in Gaul; they cross the Channel to Britain and make their way to Eboracum (York), capital of Britannia Secunda and home to a large military base. They campaign with success against the Picts.
• Galerius begins a series of campaigns against the Sarmatians, winning his first victory before the end of the year.
• 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.
• Patron of Pozzuoli, Saint Proculus, and patron of Naples, Saint Januarius are thrown to wild beasts in Pozzuoli's Flavian Amphitheater, then beheaded at Solfatara.
• In this or the following year, the Baths of Diocletian are dedicated; the thermae become the largest imperial baths in Rome.

• The Daysan River floods Edessa.

• Landowners dominate the Roman Empire, and enjoy the title of senator, which exempts them from the crushing taxes imposed on the rest of the population. The Senate has lost all its power and the landowners almost never attend Senate sessions. Members of municipal senates (curiales or decuriones) are charged with the responsibility of collecting taxes and paying arrears; smaller landowners are held responsible for providing recruits for the Roman army, and with keeping wastelands under cultivation.

• 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.
• The Council of Illiberis decrees that priests must be celibate. Additionally, it condemns visiting the homes of Jews and prohibits Christian women from marrying Jews, unless they have converted.

• 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.
• Constantine institutes toleration of the Christians in his territories.
• 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.
• Building on the efforts of Diocletian, Galerius introduces the poll tax to central and southern Italy and truncates the size of the Praetorian Guard, with plans to disband the Guard altogether.
• October 28 – Maxentius, son of former Western Emperor Maximian, leads a revolt by the Praetorian Guard and members of the Senate in Rome, and is proclaimed Emperor. Southern Italy supports Maxentius, as do Africa, Corsica, Sardinia and Sicily. Maxentius recalls Maximian from retirement, who joins his son in Rome.
• Winter: Constantine fights with success against the Franks.
• Galerius has the Rotunda of Galerius built in Thessaloniki (Macedonia).

• The War of the Eight Princes ends in China.

• The Synod of Elvira concludes with the issue of various canons, including one declaring that killing through a magic spell is a sin and the work of the devil.
• Metrophanes becomes bishop of Byzantium.
• The Patriarchate of Lisbon is established.
• Christianity is established in Roman Britain. British bishops participate in the councils of Arles (314), Nicaea (325) and Arminum (349).

• Winter: Emperor Galerius wins his second victory over the Sarmatians.
• 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.
• Summer: Anticipating an offensive by Galerius, Maximian travels to Gaul to make an alliance with Constantine I.
• Late summer or autumn: Galerius invades Italy but Maxentius remains behind the walls of Rome. Galerius finds he cannot besiege the city, and the image of an emperor making efforts against Rome hurts Galerius' image among the troops. The fact that Maxentius is his son-in-law does not help, and Maxentius makes an effort to bribe Galerius' troops. Galerius unsuccessfully attempts to negotiate, and recognizing Maxentius' attempts at bribery and the danger of being trapped in Italy by Maximian and Constantine, Galerius chooses to withdraw from Italy. To satiate his troops during the withdrawal, he pillages the Italian countryside. Meanwhile, Maxentius executes Severus.
• November 11 – Following the murder of Severus, Galerius promotes Licinius to Augustus.
• December: Constantine marries Maximian's daughter Fausta, and is promoted to Augustus by Maximian.
• Near the end of the year, Galerius gives his wife (Diocletian's daughter) Galeria Valeria the title of Augusta.

• 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.

• Winter: Emperor Galerius wins his third and final victory over the Sarmatians.
• 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.
• The overthrow of Maximian prompts the soldiers of Roman Africa to prop up the vicarius of Africa, Domitius Alexander, as a usurper.
• Constantine raids the territory of the Bructeri and builds a bridge across the Rhine at Cologne.
• November 11 – The Conference of Carnuntum: Attempting to keep peace within the Roman Empire, Galerius recalls Diocletian briefly from retirement, and they convene with Maximian. Diocletian persuades Maximian to return to retirement, and he and Galerius declare Maxentius a public enemy. Licinius is proclaimed Augustus of the west, while rival contender Constantine I is again declared Caesar.
• Bereft of his father's support, Maxentius increasingly presents himself as the Conservator Urbis Suae (Preserver of His Own City). Construction of the Basilica of Maxentius (or Basilica Nova), the largest building in the Roman Forum, is begun.
• Maxentius institutes toleration of the Christians in his territories.

• The Han-Zhao Dynasty is established in northern China, marking the official inception of the long lasting Uprising of the Five Barbarians.

• May 27 or June 26 – Pope Marcellus I succeeds Pope Marcellinus as the 30th pope.

• Gaius Ceionius Rufius Volusianus, the Praetorian Prefect of Emperor Maxentius, defeats the usurper Domitius Alexander and purges Africa of his supporters.

• 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.
• Hormizd is succeeded by his infant son Shapur II following the brief reign and murder of Adur Narseh.

• Pope Marcellus I is banished from Rome by Emperor Maxentius.
• 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).