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|421 by topic|
|Ab urbe condita||1174|
|Balinese saka calendar||342–343|
|Chinese calendar||庚申年 (Metal Monkey)|
3117 or 3057
— to —
辛酉年 (Metal Rooster)
3118 or 3058
|- Vikram Samvat||477–478|
|- Shaka Samvat||342–343|
|- Kali Yuga||3521–3522|
|Iranian calendar||201 BP – 200 BP|
|Islamic calendar||207 BH – 206 BH|
|Minguo calendar||1491 before ROC|
|Seleucid era||732/733 AG|
|Thai solar calendar||963–964|
547 or 166 or −606
— to —
548 or 167 or −605
Year 421 (CDXXI) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Agricola and Eustathius (or, less frequently, year 1174 Ab urbe condita). The denomination 421 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- 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 Lakhmids Arabs of Hirah.
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