The 420s decade ran from January 1, 420, to December 31, 429.
- The legendary Pharamond is said to have led the Franks across the Rhine.
- The Huns, under leadership of the brothers Octar and Rugila, expand their rule through neighbouring tribal groups.
- Yazdegerd I dies after a 21-year reign and is succeeded by his son Bahram V, who becomes head of the Persian Empire.
- Abdas, bishop of Susa, is accused of burning down one of the fire temples of Ahura Mazda.
- The Jin Dynasty ends in China. Liu Yu (Emperor Wu of Liu Song) becomes the first ruler of the Liu Song Dynasty. Nanjing is reinstated as the capital of northern China.
- The Southern Dynasties begin in China.
- Guisin becomes king of the Korean kingdom of Baekje.
- February 8 – Constantius III is appointed co-emperor (Augustus) with his ineffectual brother-in-law, Honorius, and becomes the real ruler of the Western Roman Empire.
- March 25 – Venice is founded at twelve o'clock noon (according to legend) with the dedication of the first church, San Giacomo, at the islet of Rialto (Italy).
- June 7 – Emperor Theodosius II marries Aelia Eudocia, a woman of Greek origin. The wedding is celebrated at Constantinople with chariot racing in the Hippodrome.
- September 2 – Constantius III dies suddenly of an illness; his wife Galla Placidia becomes, for the second time, a widow. She departs with her children Grata Honoria and Valentinian to the court of Constantinople.
- Rugila, chieftain of the Huns, attacks the dioceses of Dacia and Thrace (Balkans). Theodosius II allows Pannonian Ostrogoths to settle in Thrace, to defend the Danube frontier.
- The Franks conquer new territories in their kingdom and sack the old Roman capital Augusta Treverorum (modern Trier).
- Roman–Sassanid War: Theodosius II starts a war against the Sassanids, sending an expeditionary force under command of Ardaburius, and invades Mesopotamia.
- Autumn – Ardaburius devastates Arzanene (Armenia) and forces the Persians to retreat to Nisibis (Syria). King Bahram V allies himself with the Lakhmid Arabs of Hirah.
- End of the Roman–Sassanid War: Emperor Theodosius II signs a 100-year peace treaty with Persia after 2 years of war. He agrees a status quo ante bellum ("the state in which things were before the war"), and both parties guarantee liberty of religion in their territories.
- March 3 – Theodosius II issues a law to form provisions in peacetime. He instructs landowners leasing towers in the Theodosian Walls to assist with the build-up of emergency goods. Theodosius pays an annual tribute of 350 pounds of gold to the Huns in order to buy peace.
- Theodosius II receives a statue at Hebdomon, military parade ground on the shores of the Propontis, just outside Constantinople. On its base (fragments are now in the Istanbul Archaeology Museum), an inscription praises him as “everywhere and forever victorious.”
- The walls of Rome's Flavian Amphitheater (Colosseum) crack during an earthquake.
- The Roman army invades Gaul; they capture and execute the Frankish king Theudemeres with his family.
- Shao Di, age 16, eldest son of Wu Di, succeeds his father as emperor of the Liu Song Dynasty (China).
- September 4 – Pope Boniface I dies after a 4-year reign that was interrupted for 15 weeks, by the faction of the antipope Eulalius. He is succeeded by Celestine I as the 43rd pope.
- Approximate date – A monastic community is established at the Maijishan Grottoes.
- August 15 – Emperor Honorius, age 38, dies at Ravenna of dropsy, perhaps pulmonary edema. With no children to claim the throne, Joannes, primicerius notariorum ("chief notary", head of the civil service), seizes the throne of the Western Roman Empire, and is declared emperor. Among his supporters are Flavius Aetius, Roman general (magister militum). Joannes' rule is accepted in the dioceses of Gaul, Hispania and Italia, but not in Africa.
- Winter – Emperor Theodosius II refuses to recognize Joannes as emperor, and prepares for war. He mobilizes an expeditionary force under command of Ardaburius, and his son Flavius Aspar.
- The Pannonian Ostrogoths who have been settled in Thrace south of the Danube (see 421) by Theodosius II organize a farmers' strike. Only payment which amounts to a huge farm loan prevents them from occupying Rome.
- Theodoret becomes bishop of Cyrrhus (Syria). He converts more than 1,000 Marcionites in his diocese.
- October 23 – Emperor Theodosius II nominates his cousin Valentinian, age 5, the imperial title nobilissimus Caesar ("most noble") of the Western Roman Empire. Valentinian is betrothed to Theodosius's own daughter Licinia Eudoxia, who is only 2 years old.
- Roman usurper Joannes sends Flavius Aetius, governor of the Palace (cura palatii), to the Huns to ask for their assistance. After negotiating, he returns to Italy with a large force.
- Winter – A Roman army under the command of Ardaburius leaves Thessalonica (modern Central Macedonia) and marches for Northern Italy, where they make their base at Aquileia.
- Shao Di, age 18, is deposed by a group of high officials and succeeded by his younger brother Wen Di as emperor of the Liu Song Dynasty. Shao is exiled to Suzhou, and later killed by an assassin.
- Summer – Joannes, Roman usurper, is defeated at the fortified city of Ravenna and brought to Aquileia. After a humiliating parade on a donkey and the insults of the populace, he is executed.
- October 23 – Valentinian III, infant son of Galla Placidia, is installed as emperor (Augustus) of the Western Roman Empire. Real power is in the hands of his mother who becomes a regent.
- Flavius Aetius leads a force of Huns (60,000 men) into Northern Italy. He reaches a compromise with Placidia, in return for obtaining the rank commander-in-chief (magister militum) in Gaul.
- The Huns advance unopposed on Constantinople, but are halted by a plague that decimates their hordes (see 433).
Arts and SciencesEdit
- February 27 – The University of Constantinople is founded by emperor Theodosius II at the urging of his wife Aelia Eudocia.
- Emperor Theodosius II gives orders to destroy the buildings and pagan temples at Olympia (Greece). The statue of Zeus is brought to Constantinople.
- Flavius Aetius, Roman general (magister militum), starts a 10-year campaign against the Visigoths in southern Gaul.
- King Gunderic of the Vandals accepts the request of the Alans in Hispania to become their ruler (approximate date).
- K'inich Yax K'uk' Mo' becomes the founder of the pre-Columbian Maya civilization at Copán (modern Honduras).
- Augustine of Hippo publishes the De Civitate Dei, City of God.
- Sisinnius becomes Archbishop of Constantinople.
- Flavius Aetius, Roman general (magister militum), arrives in southern Gaul with an army (40,000 men) and defeats the Visigoths under king Theodoric I, who are besieging the strategic city of Arles.
- Bonifacius, Roman governor (Last of the Romans), revolts in Africa against emperor Valentinian III. Under the influence of Aetius, he is convicted of treason by empress-mother Galla Placidia.
- The Ephthalites (White Huns) invade Western Asia and reduce the Sasanian Empire threat to the Eastern Roman Empire. King Bahram V sends an expeditionary force into Khorasan.
- King Jangsu transfers the Goguryeo capital from Gungnae City (modern Ji'an, Jilin) on the banks of the Yalu River to Pyongyang (modern Korea).
- Biyu becomes king of the Korean kingdom of Baekje.
- Flavius Felix is elected consul for the Western Empire and issues consular diptychs during his political office.
- King Gunderic, age 49, dies after a reign of 21 years, and is succeeded by his half-brother Genseric. He is styled with the title Rex Wandalorum et Alanorum ("King of the Vandals and Alans"). Genseric increases his power and wealth in the residence of the province of Hispania Baetica (Southern Spain).
- King Vortigern invites a number of Germanic warriors to aid him in consolidating his position in Britain, according to the Historia Brittonum. He hires Saxons who are probably settled in Kent as mercenaries to fight against the Picts and the Scots beyond Hadrian's Wall.
- Chlodio, king of the Salian Franks, invades Northern Gaul and defeats the Roman army at Cambrai. He extends his kingdom south to the River Somme and makes Tournai (modern Belgium) his residence. Frankish expansion changes the borders, founding Francia.
- Artaxias IV, last king of Greater Armenia, is deposed by Bahram V. The Arshakuni Dynasty ends and the kingdom becomes a province of the Persian Empire.
- April 10 – Nestorius is made patriarch of Constantinople. He preaches a new doctrine that will be called Nestorianism. It makes a distinction between the divine and human natures of Jesus but comes under immediate attack from pope Celestine I and Cyril of Alexandria.
- Hydatius becomes bishop of Aquae Flaviae in Gallaecia (modern Chaves) in Portugal.
- John succeeds Theodotus as patriarch of Antioch, and gives his support to Nestorius.
- Euthymius the Great builds a monastery in Palestine, near the Dead Sea.
- Spring – The Vandals, led by Genseric ("Caesar King"), invade North Africa. They land with a force of 80,000 men including Alans, and Germanic tribesmen with their families from the Iberian Peninsula, across the narrow Strait of Gibraltar. The Vandal fleet raids the coasts of the Mediterranean Sea, and blockades the grain and oil supply to Italy.
- Genseric seizes lands from the Berbers and destroys church buildings all over Mauretania. He goes on a rampage forcing Bonifacius, Roman governor, to retreat to the fortified coastal town of Hippo Regius (modern Annaba).
- Bonifacius, weakened by the civil war against empress Galla Placidia, sues for peace and is elevated to the rank of supreme commander (magister militum) of Africa.
- Emperor Theodosius II starts to reform the Codex Theodosianus in Constantinople. He establishes a committee to codify all Roman laws. All funds raised by Jews to support schools have to be turned over to the state treasury.
- The Temple of Goddess Athena on the Acropolis of Athens is sacked. Athenian Pagans are persecuted.
- A Gaulish assembly of bishops dispatch Germanus of Auxerre and Lupus of Troyes to Britain to visit the island. To satisfy pope Celestine I they combat the Pelagian heresy.
- Hilary succeeds his kinsman Honoratus and becomes archbishop of Arles.
- Domnus II, future patriarch of Antioch, is ordained as a deacon.
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- Anthemius, emperor of the Western Roman Empire (approximate date)
- Ecdicius, Roman general (magister militum) (approximate date)
- Glycerius, emperor of the Western Roman Empire (approximate date)
- Libius Severus, emperor of the Western Roman Empire (approximate date)
- Majorian, emperor of the Western Roman Empire (approximate date)
- Palladius, caesar and son of Petronius Maximus (approximate date)
- Valamir, king of the Ostrogoths (approximate date)
- Yuan Can, high official of the Liu Song Dynasty (d. 477)
- August 8 – Casper, ruler of the Maya city of Palenque
- Genevieve, patron saint (approximate date)
- Licinia Eudoxia, Roman empress (d. 493)
- Erbin of Dumnonia, Brythonic king (approximate date)
- Qi Gaodi, Chinese emperor of the Southern Qi Dynasty (d. 482)
- Wang Xianyuan, empress and wife of Song Xiaowudi (d. 464)
- January 21 – Yazdegerd I, king of the Sassanid Empire
- February 26 – Saint Porphyry, bishop of Gaza (Palestine)
- September 30 – Saint Jerome, priest and translator of the Bible
- Saint Abdas, bishop of Susa (Iran)
- Li Xin, duke of the Chinese state Western Liang
- Orosius, Christian historian and theologian (approximate date)
- Pelagius, British monk (approximate date)
- Yao, empress consort and wife of Mingyuan
- September 2 – Constantius III, emperor of the Western Roman Empire
- Jin Gongdi, last emperor of the Jin Dynasty (b. 386)
- Li Xun, ruler of the Chinese state Western Liang
- Mary of Egypt, patron saint (approximate date)
- Ravina I, rabbi (teacher) and Jewish Talmudist
- September 4 – Pope Boniface I
- Abraham of Cyrrhus, Syrian hermit and bishop
- Fa-Hien, Chinese Buddhist monk and traveler (approximate date)
- Theudemeres, king of the Franks (approximate date)
- Wu Di, emperor of the Liu Song Dynasty (b. 363)
- August 15 – Honorius, Roman Emperor (b. 384)
- December 23 – Ming Yuan Di, ruler of the Xianbei state Northern Wei (b. 392)
- Eulalius, antipope of Rome
- Tufa, Chinese princess and wife of Qifu Chipan
- Xiao Wenshou, empress dowager of the Liu Song Dynasty (b. 343)
- November 5 – Atticus, archbishop of Constantinople
- Gamliel VI, last Nasi (head of the Sanhedrin)
- Helian Bobo, emperor of the Chinese Xiongnu state Xia (born 381)
- Joannes, Roman usurper
- Mavia, Arab warrior-queen
- Sulpicius Severus, Christian writer (approximate date)
- Yax Nuun Ayiin I 15th Ajaw of Tikal (approximate date)
- Fu Liang, official of the Liu Song Dynasty (b. 374)
- Xie Hui, general of the Liu Song Dynasty (b. 390)
- Xu Xianzhi, official of the Liu Song Dynasty (b. 364)
- Zhang, empress dowager of the Liu Song Dynasty
- December 24 – Sisinnius I, archbishop of Constantinople
- Guisin, king of Baekje (Korea)
- Tao Qian, Chinese poet of the Eastern Jin Dynasty (b. 365)
- Gunderic, king of the Vandals and Alans (b. 379)
- Qifu Chipan, prince of the Xianbei state Western Qin
- Theodore of Mopsuestia, bishop and theologian
- Bernard Grun, The Timetables of History, Simon & Schuster, 3rd ed, 1991. ISBN 0671749196
- "List of Rulers of Korea". www.metmuseum.org. Retrieved 20 April 2019.
- The End of Empire (p. 87). Christopher Kelly, 2009. ISBN 978-0-393-33849-2
- "Colosseum". Ancient History Encyclopedia. Retrieved 2020-09-27.
- MacDowall, Simon (2018). The Franks. London: Grub Street Publishers. p. 19. ISBN 978-1-4738-8960-6.
- Memoirs of Eminent Monks.
- Smith, Sir William (1849). Dictionary of Greek and Roman biography and mythology. 3. C.C. Little and J. Brown. p. 1211.
- Urbainczyk, Theresa (2002). Theodoret of Cyrrhus: the bishop and the holy man. University of Michigan Press. p. 21. ISBN 978-0-472-11266-1.
- Retief, F. P.; Cilliers, L. (January 1998). "The epidemic of Athens, 430-426 BC". South African Medical Journal = Suid-Afrikaanse Tydskrif vir Geneeskunde. 88 (1): 50–53. ISSN 0256-9574. PMID 9539938.
- "List of Rulers of Korea". www.metmuseum.org. Retrieved 20 April 2019.
- "Constantius III | Biography & Facts". Encyclopedia Britannica. Retrieved 17 March 2019.
- "Eulalius | antipope". Encyclopedia Britannica. Retrieved 29 March 2020.