The 300s decade ran from January 1, 300, to December 31, 309.
- The Franks penetrate into what is now northern Belgium (approximate date).
- The city of Split is built.
- The Camp of Diocletian is built in Palmyra.
- A Romano-Celtic temple-mausoleum complex is constructed in what is now Lullingstone, and also in Anderitum (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 (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.
Arts and sciences
- 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.
- Tiridates III makes his kingdom of Armenia the first state to adopt Christianity as its official religion.
- Approximate date of the Synod of Elvira in Elvira, Spain, which prohibits interaction with Jews, pagans, and heretics.
- Emperor Diocletian issues his Edict on Maximum Prices, which, rather than halting rampant inflation and stabilizing the economy, over time adds to inflationary pressures, by flooding the economy with new coinage, and by setting price limits too low.
- Diocletian begins the construction of new roads in the Roman Empire. The Strata Diocletiana is built, and lined with a series of forts (quadriburgia); it runs from the Gulf of Aqaba (Arabia) to the Euphrates.
- King Tiridates III of Armenia proclaims Christianity as the official state religion, making Armenia the first nation to adopt Christianity as its official religion. Construction of the original Etchmiadzin Cathedral by Gregory the Illuminator begins.
- Emperor Diocletian begins passing laws against Christians and a policy of religious oppression in Antioch.
- Narseh, Shahanshah of the Sassanid Empire, dies after a 9-year reign. He is succeeded by his son Hormizd II.
Arts and sciences
- Great Persecution: Emperor Diocletian launches the last major persecution of Christians in the Empire. Hierocles is said to have been the instigator of the fierce persecution of the Christians under Galerius. They are forbidden to worship in groups, and thousands of them are killed in the next 10 years.
- February 24 – Galerius publishes his edict that begins the persecution of Christians in his portion of the Empire.
- September 25 – On a voyage preaching the gospel, Saint Fermin of Pamplona is beheaded in Amiens, France.
- November 20 – Diocletian makes a visit to Rome. The Augusti and the Caesars are united for the first time to celebrate the 20th anniversary of Diocletian's accession.
- The Arch of Galerius is dedicated in Thessaloniki.
- Etchmiadzin Cathedral is completed by Gregory the Illuminator and Tiridates III, king of Armenia.
- January 6 – baptization of Tiridates III of Armenia.
- Emperor Diocletian issues four edicts aimed at destroying Christianity; churches are to be dismantled, clergymen arrested, and their followers forced to sacrifice to pagan gods on pain of death. 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.
- Constantius Chlorus defeats the Alemanni and fortifies the town of Konstanz (Germany).
- Diocletian fights a campaign against the Carpi on the Danube and becomes seriously ill.
- A Triumphal Arch is built for Diocletian in Rome.
- Sixteen Kingdoms: The Wu Hu uprising establishes the Han Kingdom, under Liu Yuan.
- Sichuan earns its independence from China.
- 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 at age 60, 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 (southern Italy), to live a life of ease in his luxury villas.
- Constantius Chlorus and Galerius are declared Augusti; Flavius Valerius Severus and Maximinus Daza are appointed Caesars.
- In the Western Empire, the capital is moved from Rome to Milan. Constantius Chlorus requests leave for his son Constantine I, who remains at Galerius's court in Nicomedia, as a virtual hostage.
- Summer – Constantine I joins his father in Gaul, from Bononia (Boulogne); they cross the Channel to Britain and make their way to Eboracum (York), capital of Britannia Secunda and home to a large military base, in order to deal with a rebellion by the Picts.
- Maximinus Daza (305–313) 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.
- 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.
- Constantius Chlorus undertakes a punitive expedition against the Picts beyond the repaired Hadrian's Wall. His son Constantine joins him on campaign and they win a brilliant victory.
- July 25 – Constantius Chlorus dies outside Eboracum (modern-day York). Constantine, age 26, is declared Augustus (emperor) by his troops. Eventually Emperor Galerius grants Constantine the title Caesar and elevates Flavius Valerius Severus (Severus II) to co-emperor of the Western Roman Empire.
- October 28 – Maxentius, son of the former Western Emperor Maximian, joins a revolt by the Praetorian Guard in Rome, and is proclaimed Emperor.
- Southern Italy, bitter over the subjection to taxation, supports Maxentius, as do Africa, Corsica, Sardinia and Sicily.
- Galerius sends Severus to suppress the rebellion in Rome. The soldiers desert him and Severus flees to Ravenna.
- The Franks cross the Rhine, but are repulsed by Constantine.
- The Baths of Diocletian are dedicated; the thermae become the largest imperial baths in Rome.
- Galerius has the Rotunda of Galerius built in Thessaloniki (Macedonia).
- Probable date – 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).
- Constantine is made a Caesar by Galerius and returns to Britain.
- March 31 – After divorcing his wife Minervina, Constantine the Great marries Fausta Flavia Maxima, the daughter of the retired co-emperor Maximian.
- Constantine I establishes his capital in Augusta Treverorum (Trier). He begins a major expansion of the city, strengthening the walls with fortified gates, and building a palace complex and the Imperial Baths.
- September 16 – Severus II is captured and imprisoned at Tres Tabernae. After Emperor Galerius unsuccessfully invades Italy to suppress Maxentius, he is executed (or forced to commit suicide).
- January 8 – Emperor Jin Huidi 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. Jin Huaidi, age 23, succeeds his father and becomes third emperor of the Jin Dynasty.
- April – History of Trier: Roman usurper Maxentius banishes his father, Maximian, who flees to the court of Constantine the Great in modern Trier.
- November 11 – The Congress of Carnuntum: Attempting to keep peace within the Roman Empire, the leaders of the Tetrarchy declare Maxentius a public enemy, and Licinius is proclaimed Augustus, while rival contender Constantine I is declared Caesar of Britain and Gaul.
- Under Constantine I, the Romans defeat the Germans along the Rhine frontier.
- Domitius Alexander is acclaimed emperor (against Maxentius) in Carthage; the African provinces come under his rule. This is a dangerous situation, because Rome depends on the African grain supply.
- Construction of the Basilica of Maxentius (Basilica Nova), the largest building in the Roman Forum, is begun.
- The Han Zhao Dynasty is established in northern China, marking the official inception of the long lasting Uprising of the Five Barbarians.
- The Spanish provinces revolt from the control of Maxentius, acknowledging Constantine the Great as their Emperor.
- A plague that may be related to anthrax spreads across the Roman Empire, causing a drastic decline in the population.
- King Hormizd II, Shah of the Persian Empire, dies after a 7-year reign and a power struggle for the succession ensues. He is succeeded by his son Shapur II.
- Pope Marcellus I is banished from Rome by the Emperor Maxentius.
- April 18 – Pope Eusebius succeeds Marcellus as the 31st pope, but is himself banished on August 17 to Sicily (These events may have taken place in 310).
In Yamato (Japan), the Kofun period dominated during this decade. It was an animistic culture which existed prior the introduction of Buddhism. A legend of the 4th century Prince Yamato Takeru alludes to the borders of the Yamato and battlegrounds in the area. A frontier was obviously somewhere close to the later Izumo province (the eastern part of today's Shimane prefecture). Another frontier, in Kyūshū, was apparently somewhere north of today's Kumamoto prefecture. The legend specifically states that there was an eastern land in Honshū "whose people disobeyed the imperial court", against whom Yamato Takeru was sent to fight. That rivalling country may have been located rather close to the Yamato nucleus area itself, or relatively far away. The today Kai province is mentioned as one of the locations where prince Yamato Takeru sojourned in his said military expedition.
Northern frontier of this age was also explained in Kojiki as the legend of Shido Shōgun's (四道将軍: Shōguns to four ways) expedition. Out of four shōguns, Ōbiko set northward to Koshi and his son Take Nunakawawake set to eastern states. The father moved east from northern Koshi while the son moved north on his way, and they finally met at Aizu (current western Fukushima). Although the legend itself is not likely to be a historical fact, Aizu is rather close to southern Tōhoku, where the north end of keyhole kofun culture as of late 4th century is located.
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- Asanga, founder of the Yogacara of Mahayana Buddhism (approximate date)
- Frumentius, Syrian Christian trader (approximate date)
- Hilary of Poitiers (approximate date)
- Li Shou, emperor of Cheng Han (d. 343)
- Jin Mindi, emperor of the Jin Dynasty (d. 318)
- Macarius of Egypt, Christian hermit and monk (approximate date)
- Zeno of Verona, Bishop of Verona and martyr (approximate date)
- approximate date – Aemilia Hilaria, Gallo-Roman physician (d. 363)
- Magnentius, Roman usurper (d. 353)
- Wang Xizhi, Chinese calligrapher (d. 361)
- Xun Guan, Chinese female warrior
- Saint Damasus, pope (366—383)
- Approximate date – Ephrem the Syrian, Syriac theologian and hymnodist
- Jia Nanfeng, empress and wife of Jin Huidi (b. 257)
- Liu Ling, Chinese scholar and poet (b. 221)
- Sima Yu, crown prince during the Jin Dynasty (b. 278)
- Sporus of Nicaea, astronomer and mathematician (approximate date)
- Theonas, Patriarch of Alexandria
- Zhang Hua, official of the Jin Dynasty (b. 232)
- Acacius of Byzantium, centurion in the Roman army and martyr
- Anthimus of Rome
- Cao Huan, last emperor of the Kingdom of Wei (b.246)
- Cessianus, Roman Catholic saint and martyr
- Crescentinus, patron saint of Urbino
- Saint Cyriacus
- Saint Devota
- Erasmus of Formiae
- Expeditus, Roman commander and martyr
- Felix and Adauctus, Christian martyrs
- Saint Fermin, Roman Catholic saint
- Lu Ji, Chinese writer and literary critic (b.261)
- Saint Pantaleon
- Romanus of Caesarea, martyr
- Victor Maurus, Christian martyr
- Vitus, Christian saint from Sicily
- October 25 – Pope Marcellinus
- December 25 – Saint Anastasia (martyred)
- Date unknown
- Saint Afra (martyred by fire)
- Saint Agape, Chionia, and Irene (martyred)
- Saint Agnes (martyred)
- Saint Alban (possibly 309)
- Bunseo of Baekje, king of Baekje (Korea)
- Saint Florian (martyred)
- Saint Gorgonius of Nicomedia (martyred)
- Saint Juliana of Nicomedia (martyred)
- Saint Lucie of Syracuse (martyred)
- Saint Margaret (martyred)
- Saint Pancras (martyred)
- Saint Philomena (martyred)
- Sima Ai, Chinese prince of the Jin dynasty (b. 277)
- Saints Theodora and Didymus (martyred)
- Saint Vincent of Saragossa (martyred)
- Januarius, patron saint of Naples
- Saint Catherine of Alexandria, martyr and virgin
- Hierocles, proconsul of Bithynia who instigated the persecution of the Christians under Galerius
- The beheading of Saint Panteleimon the Great Martyr & Healer
- Porphyry, Neoplatonist philosopher
- Proculus of Pozzuoli, patron saint of Pozzuoli
- Tuoba Yituo, chieftain of the Tuoba clan (China)
- Wang Rong, politician of the Jin dynasty (b. 234)
- February 17 – Theodore of Amasea, Roman soldier and Christian martyr
- March 4 – Adrian and Natalia of Nicomedia, Christian martyrs
- July 25 – Constantius Chlorus, Roman emperor (b. c.250)
- August 25 – Saint Maginus, Christian hermit and martyr from Tarragona, Catalonia; beheaded
- January 8 – Jin Huidi, Chinese Emperor of the Jin Dynasty (b. 259)
- September 16 – Severus II, deposed Roman Emperor (murdered)
- Saint Alban (possible date—also 304)
- Saint Elias and companions
- Hormizd II, king of the Persian Empire
- Pope Marcellus I
- "List of Rulers of Korea". metmuseum.org. Retrieved 19 April 2019.
- C.W. Dugmore, The Journal of Ecclesiastical History (Cambridge University Press) p.268.
- A. Dzh. (Arman Dzhonovich) Kirakosian, The Armenian Massacres, 1894–1896: 1894–1896 : U.S. media testimony, p.131.
- "OrientalOrthodox.org". Archived from the original on July 27, 2011. Retrieved March 4, 2010.
- Johann Christian Wilhelm Augusti, Georg Friedrich Heinrich Rheinwald, Carl Christian Friedrich Siegel, The Antiquities of the Christian Church p.466.
- "List of Rulers of Korea". www.metmuseum.org. Retrieved 20 April 2019.
- Vagi, David (2016). Coinage and History of the Roman Empire. Routledge. p. 476. ISBN 978-1-135-97125-0.
- "Lu Ji's (261–303) Essay on Literature dated 1544 and 1547". www.metmuseum.org. Retrieved 5 January 2020.
- Knechtges, David R.; Chang, Taiping (2010). Ancient and Early Medieval Chinese Literature (vol.I): A Reference Guide, Part One. BRILL. p. 542. ISBN 9789004191273.