Geopolitics & Diplomacy in the 4th century

71 sourced entries, 300 CE to 399 CE, oldest first. Also see Geopolitics & Diplomacy across every century or everything else in the 4th century.

• 300Emperor 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.source
• 301September 3 – The republic of San Marino is established (approximate date).source
• 303November 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.source
• 304Cheng-Han earns its independence from Jin dynasty.source
• 304Sixteen Kingdoms: The Xiongnu establishes the Han-Zhao state under Liu Yuan, often seen as the start of the Upheaval of the Five Barbarians.source
• 305May 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.source
• 305Constantius 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.source
• 305Constantius I and Galerius are declared Augusti; Flavius Valerius Severus and Maximinus Daza are appointed Caesars.source
• 306October 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.source
• 306Constantine institutes toleration of the Christians in his territories.source
• 306Building 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.source
• 306Constantine 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.source
• 311Fearing an alliance between Licinius and Constantine I, Maximinus forges a secret alliance with Emperor Maxentius.source
• 313Nintoku, the fourth son of Ōjin, becomes the 16th emperor of Japan. The historical profile of Nintoku is generally accepted as fact without attributing all of the things he allegedly accomplished.source
• 318Liu Yao becomes the new emperor of Han-Zhao and moves his capital to Chang'an.source
• 319Emperor Constantine the Great prohibits the separation of the families of slaves, during a change in ownership.source
• 320Crispus, eldest son of Constantine I, leads a victorious campaign against the Franks, assuring twenty years of peace along the Rhine frontier. He establishes his residence in Augusta Treverorum (modern Trier), capital of Germania.source
• 324January 1 – Flavius Julius Crispus Caesar, the sons of the Emperor Constantine and his expected heir, and Flavius Claudius Constantinus begin their one year terms as the new Roman consuls.source
• 324December 19 – Licinius abdicates his position as Emperor. He is pardoned by Constantine I as a result of the supplication of his wife Constantia (who is Constantine's halfsister), and banished to Thessalonica as a private citizen.source
• 324(Date unknown) The Roman Emperor Constantine I seizes the Byzantine Empire's capital, Byzantium, and commences work on rebuilding the city as the Eastern Empire's capital, which he will inaugurate as Constantinople in 330.source
• 325German and Sarmatian campaigns of Constantine: Emperor Constantine I personally assures the security of the Danube frontier by defeating the Goths, the Vandals and the Sarmatians.source
• 330May 11 – Emperor Constantine the Great dedicates the Column of Constantine and Constantinople, or Nova Roma (modern-day Istanbul). He had spent four years expanding the city of Byzantium, having chosen the site for its strategic location on the Bosporus. The city became the capital of the Byzantine Empire.source
• 331Gogugwon becomes ruler of the Korean kingdom of Goguryeo.source
• 333Flavius Dalmatius and Domitius Zenofilus are appointed consuls.source
• 334Emperor Constantine the Great reauthorises gladiatorial combat.source
• 335September 19 – Flavius Dalmatius is raised to the rank of Caesar, with control of Thracia, Achaia, and Macedonia.source
• 335Tuoba Hena ousts Tuoba Yihuai as chieftain of the Tuoba Clan.source
• 336The military successes of Emperor Constantine I result in most of Dacia being reconquered by the Roman Empire.source
• 337Murong Huang claims the title of Prince of Yan, effectively beginning the kingdom of Former Yan.source
• 341Constans I begins a successful campaign against the Franks.source
• 342Constans I campaigns victoriously against the Franks.source
• 345The Kadamba Dynasty is founded by Mayurasharma.source
• 349Shi Shi, youngest of Shi Hui's sons, reigns for 33 days before being deposed and executed at age 10 by Shi Zun. After a brief reign of 183 days, Emperor Shi Zun and his mother Empress Zheng Yingtao are executed; his brother Shi Jian succeeds him, only to be toppled after 103 days in early 350 by Shi Zhi,source
• 350March 1 – Vetranio is asked by Constantina, sister of Constantius II, to proclaim himself Caesar. Constantius accepts the new emperor and sends him funds to raise an army.source
• 350December 25 – Vetranio meets Constantius II at Naissus (Serbia) and joins forces with him. Vetranio is forced to abdicate his title, and Constantius allows him to live as a private citizen on a state pension.source
• 350King Pushyavarman establishes the Varman Dynasty in Assam.source
• 351Fú Jiàn, an ethnic Di, declares himself "Heavenly Prince" (Tian Wang) during the collapse of Later Zhao, and establishes Former Qin.source
• 354The Bulgars are first mentioned in extant European chronicles.source
• 357The reign of Fú Jiān, the emperor of Former Qin, commences in China.source
• 358The Salian Franks capitulate to Julian the Apostate in Gaul. He allows them to form a Roman foederati in Toxandria. Frankish settlers are established in areas in the north and the east to help with the defense of the Rhine frontier.source
• 358Emperor Constantius II builds new forts to secure upper Mesopotamia. Persia's king Shapur II sends an emissary to Constantinople with gifts and a letter wrapped in white silk. He requests that Constantius return the lands of his ancestors from the Euphrates to the frontier of Macedonia. Constantius tactfully refuses to cede any territories.source
• 360Emperor Constantius II and Julian exchange several letters, both hoping to avoid a civil war.source
• 361July 10 – Sixteen Kingdoms: Jin Aidi, age 20, succeeds Jin Mudi, as emperor of the Eastern Jin Dynasty.source
• 364February 26 – Valentinian I is proclaimed Emperor by officers of the Roman army at Nicaea in Bithynia. He addresses the soldiers (who threaten to riot) in a speech. He founds the Valentinianic dynasty and rules the Western portion of the Roman Empire, from Caledonia (Scotland) to the Rhine frontier, ensuring it a few years of relative security. He settles in Lutetia (Paris) and establishes a militia to defend the region.source
• 365September 28 – Procopius revolts and bribes two legions passing by Constantinople. He proclaims himself Emperor, and takes control of Thrace and Bithynia.source
• 367The first Korean envoy arrives in Japan, emissary of the government of Kudara.source
• 367Eunomius of Cyzicus is banished to Mauretania for harbouring the usurper Procopius.source
• 369Spring – Emperor Valens crosses the Danube, and attacks the Gothic tribes (Greuthungi and Tervingi). Valens and Athanaric, the Gothic king, eventually sign a treaty.source
• 369Theodosius the Elder brings Britain fully back to the Empire after the Great Conspiracy of 367.source
• 372Athanaric starts building new defensive works to protect his people against the Alans and the Huns.source
• 372The first diplomatic ties are established between the Korean kingdom of Baekje and the Chinese court of the Jin Dynasty.source
• 373The Aqueduct of Valens is inaugurated near Constantinople (modern Istanbul); the aqueduct has a length of 971 meters.source
• 375Emperor Chandragupta II becomes ruler of the Gupta Empire (India). He is the son of Samudragupta the Great and retains his reign by an aggressive expansionist policy.source
• 375The Quadi accept an uneasy peace from Merobaudes (Magister militum), which gives them land to settle on the Danube.source
• 375Gratian, age 16, takes over the government at Augusta Treverorum (modern Trier), but ministers wishing to retain the loyalty of the Illyrian army fear a usurper. They proclaim Valentinian's 4-year-old son Valentinian II co-emperor with his mother, Justina, as regent. Gratian reserves for himself the administration of the Gallic provinces, and hands over Italy, Illyrium, Hispania and Africa to his stepmother, who makes Mediolanum (Milan) her residence.source
• 376China is divided between the Former Qin in the north and the Eastern Jin in the south.source
• 379January 19 – Emperor Gratian elevates Flavius Theodosius at Sirmium, giving him the title Augustus with power over all the eastern provinces. Theodosius comes to terms with the Visigoths and settles them in the Balkans as military allies (foederati).source
• 379Gratian refuses the title of Eastern Emperor.source
• 382October 3 – Emperor Theodosius I commands his general Saturninus to conclude a peace treaty with the Visigoths, allowing them to settle south of the Danube. They are installed as foederati in Moesia and Thrace with the title of "Allies of the Roman People", in exchange for furnishing a contingent of auxiliary troops to defend the borders.source
• 383January 19 – Arcadius is elevated to Emperor.source
• 383Niall of the Nine Hostages becomes the first High King of Ireland.source
• 383Roman troops in Britannia proclaim Magnus Maximus as their Emperor. He crosses over to the continent and makes Trier his capital. Gaul, the Italian provinces and Hispania proclaim loyalty to him.source
• 383Theodosius I sends Flavius Stilicho as an envoy to the Persian court of King Shapur III at Ctesiphon, to negotiate a peace settlement relating to the partition of Armenia.source
• 384Gogugyang becomes ruler of the Korean kingdom of Goguryeo.source
• 384King Shapur III signs the Peace of Acilisene with Theodosius I. Armenia is divided in two kingdoms and becomes a vassal state of the Roman Empire and Persia. The friendly relations survive for 36 years.source
• 386Emperor Theodosius I signs a peace treaty with King Shapur III; they divide Armenia into two kingdoms (vassal states). The treaty establishes friendly relations between the Roman Empire and Persia for the next 36 years.source
• 386A column is constructed at Constantinople in honour of Theodosius I. Reliefs depict the emperor's victory over the barbarians in the Balkan.source
• 387Peace of Acilisene: King Shapur III signs a treaty with Theodosius I. Armenia is divided in two kingdoms, and becomes a vassal state of the Roman Empire and Persia.source
• 398Revolt of Alaric I: After Stilicho returns to Italy, the Eastern Roman Empire concludes a peace treaty with Alaric. The Visigoths are given a settlement area in Illyricum and their king is appointed magister militum per Illyricum.source
• 398An imperial edict obliges Roman landowners with plantations to yield 1/3 of their fields to the "barbarians" who have been settled in the Roman Empire.source
• 399Gainas, a Gothic leader, is made magister militum and forms an alliance with deserters of Tribigild along the Bosphorus. He proclaims himself co-regent (usurper), and installs his forces in Constantinople. Gainas deposes anti-Gothic officials and has Eutropius, imperial advisor (cubicularius), executed.source