The 290s decade ran from January 1, 290, to December 31, 299.
- Emperors Diocletian and Maximian meet in Milan, on the 5-year anniversary of their rule, to discuss politics and war. Rome becomes a ceremonial capital.
- Carausius, who has established himself as king of Britain, is also reluctantly acknowledged by Diocletian and Maximian as third emperor. During his reign, he defeats Frankish and Saxon raids on the English coast.
- Carausius begins to build a series of fortifications on the Saxon Shore, in south-east England.
- May 16 – Emperor Jin Wudi, founder of the Western Jin Dynasty, dies after a 25-year reign. He reunifies north and south, but gives away many dukedoms to his kinsmen. Crown Prince Jin Huidi succeeds his father, and has to deal with conflicts among the aristocratic families in China.
- War of the Eight Princes: After the death of Emperor Sima Yan (Jin Wudi), a civil war breaks out among the princes and dukes of the Jin Dynasty. The struggle devastates and depopulates the provinces of northern China.
- Achilleus, Roman general, is proclaimed emperor in Alexandria. For two years he rules over Egypt, but in the end the rebellion is crushed by Emperor Diocletian.
- March 1 – Emperors Diocletian and Maximian appoint Constantius Chlorus and Galerius as Caesars. This is considered the beginning of the Tetrarchy, known as the Quattuor Principes Mundi ("Four Rulers of the World").
- The four Tetrarchs establish their capitals close to the Roman frontier:
- Diocletian's Palace is built at a small bay on the Dalmatian coast, four miles from Salona, today's Split, Croatia.
- Constantius Chlorus retakes some of the Gallic territories and conquers the crucial port of Bononia (modern Boulogne).
- Carausius, Roman usurper, is murdered by his finance minister Allectus, who proclaims himself "emperor" of Britain.
- Constantius Chlorus defeats the Franks on the Rhine frontier in Batavia (Netherlands).
- King Bahram II of the Persian Empire dies after a 17-year reign; his son Bahram III ascends to the throne. After four months, he is murdered by viceroy Narseh, with support of the nobility.
- Narseh becomes king of Persia, and engages Rome in eight years of constant warfare.
- Probus succeeds Rufinus, as Patriarch of Constantinople.
- Galerius, Roman Caesar in the Balkans, proves his worth in campaigning on the Danube frontier, fighting the Goths, Marcomanni, Sarmatians, and Carpi.
- Galerius is given the job of land reclamation and repopulation, moving the entire tribe of the Carpi to settlements within the Roman Empire.
- Emperor Diocletian goes, with the young Constantine I the Great (later the first Christian Roman Emperor) on his staff, to Egypt. He besieges Alexandria, and deposes "emperor" Achilleus.
- Galerius, Roman Caesar in the Balkans, is dispatched to Egypt to fight against the rebellious cities Busiris and Coptos.
- Tuoba Luguan divides the territory of the Tuoba clan into three areas. His nephews Tuoba Yilu and Tuoba Yituo become chieftains of the western and central areas of (Shanxi province). Tuoba Luguan dominates the eastern area (near Hohhot).
- Petra rejoins the province of Palestine, and is converted to Christianity by the Syrian monk Barsauma.
- Constantius Chlorus assembles two invasion fleets with the intent of crossing the English Channel. The first is under the command of Asclepiodotus, Constantius' long serving Praetorian Prefect. He sails from the mouth of the Seine, and lands near the Isle of Wight, where his forces defeat the usurper Allectus in Hampshire. Constantius leaves Boulogne with his fleet, and occupies London, saving the city from an attack by Frankish mercenaries who are roaming the province.
- Maximian, emperor (Augustus) of the Roman Empire, mobilises an army, consisting of Praetorian cohorts, Aquileian, Egyptian, and Danubian legionaries, marching through Spain. He crosses the Strait of Gibraltar into Mauretania (modern Morocco) to protect the area against Frankish pirates.
- Constantius Chlorus reconquers Britain; he rebuilds the cities Eboracum (York), Londinium (London), and Verulamium (St Albans).
- Emperor Diocletian dispatches his son-in-law Galerius with a large army to Armenia. He invades Mesopotamia, but suffers a complete defeat outside Ctesiphon against the Persian king Narseh, and is forced to retreat. Galerius crosses the Euphrates into Syria to join Diocletian at Antioch.
- April 22 – Pope Caius dies at Rome after a 13-year reign and is succeeded by Pope Marcellinus as the 29th pope.
- Emperor Maximian begins an offensive against the Berbers in Mauritania, driving them back into their homelands in the Atlas Mountains. He spends the rest of the winter in Carthage (Africa).
- Galerius makes preparations in Syria for a campaign against the Persian king Narseh. He recruits veterans from Illyria and Moesia, and strengthens his bodyguard with Gothic auxiliaries.
- Battle of Lingones: Constantius Chlorus defeats the Alamanni in the territory of the Lingones (Langres) in Gaul. He strengthens the border along the Rhine frontier.
- Battle of Vindonissa: Constantius I defeats the Alamanni at Vindonissa (modern Switzerland).
- March 10 – Emperor Maximian concludes his campaign in North Africa against the Berbers, and makes a triumphal entry into Carthage.
- The Baths of Diocletian are commissioned by Maximian in honor of Emperor Diocletian.
- Christians are expelled from the Roman army.
- Galerius invades Armenia with an army of 25,000 men. He makes personal reconnaissances, and marches deep into occupied Mesopotamia.
- Battle of Satala: Galerius decisively defeats king Narseh. He captures the Persian camp, including Narseh's family, harem and treasure.
- King Tiridates III is restored as ruler of Armenia.
- The manufacture of cultured silk becomes popular from Korea to Japan.
- Bunseo becomes king of the Korean kingdom of Baekje.
- Girim becomes the king of the Korean kingdom of Silla.
- Peace of Nisibis: Galerius signs a treaty with the Persian king Narseh that will last for 40 years. The Persians accept Roman dominion over Armenia and northern Mesopotamia. The Tigris becomes the boundary between Rome and the Sassanid Empire.
- Galerius commissions the Arch of Galerius in Thessaloniki (Greece). The structure is built to celebrate the war and victory over the Sassanid Persians.
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- Saint Agnes, Christian martyress (d. c. 304)
- Saint Hilarion, anchorite and saint (d. 371)
- Li Xiu, female general during the Jin Dynasty
- Saint Philomena, Christian martyress (d. c. 304)
- Sima Liang, regent during the reign of Sima Yan
- Sima Wei, prince during the Jin Dynasty (b. 271)
- Wei Guan, general of the Kingdom of Wei (b. 220)
- Wen Yang, general of the Kingdom of Wei (b. 238)
- Yang Jun, official during the reign of Sima Yan
- Bahram II, king of Persia
- Bahram III, king of Persia
- Carausius, Roman usurper of northern Gaul and Britain
- Tuoba Chuo, chieftain of the Chinese Tuoba tribe
- Chen Shou, author of the San Guo Zhi (b. 233)
- Thirumalisai Alvar, one of the 12 Azhwar Saints who lived for 4500 Years. (b. 4203 BCE)
- Zhou Chu, Jin dynasty general, son of Zhou Fang (b. 236)
- King Chaekgye of Baekje
- Saint Marcellus of Tangier, Christian martyr
- Cassian of Tangier, Christian saint
- Yurye of Silla, ruler of the Korean state of Silla
- "List of Rulers of Korea". www.metmuseum.org. Retrieved 18 April 2019.
- Spencer C. Tucker (2009-12-23). A Global Chronology of Conflict: From the Ancient World to the Modern Middle East: From the Ancient World to the Modern Middle East. ABC-CLIO. p. 153. ISBN 978-1-85109-672-5.
- "List of Rulers of Korea". www.metmuseum.org. Retrieved 20 April 2019.
- Giaquinta, Mariano; Modica, Giuseppe (2012). Mathematical Analysis: Functions of One Variable. Springer Science & Business Media. p. 315. ISBN 978-1-4612-0007-9.
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