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|Millennium:||1st millennium BC|
|451 BC by topic|
|Gregorian calendar||451 BC
|Ab urbe condita||303|
|Ancient Egypt era||XXVII dynasty, 75|
|- Pharaoh||Artaxerxes I of Persia, 15|
|Ancient Greek era||82nd Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||己丑年 (Earth Ox)
2246 or 2186
— to —
庚寅年 (Metal Tiger)
2247 or 2187
|Coptic calendar||−734 – −733|
|Ethiopian calendar||−458 – −457|
|- Vikram Samvat||−394 – −393|
|- Shaka Samvat||N/A|
|- Kali Yuga||2650–2651|
|Iranian calendar||1072 BP – 1071 BP|
|Islamic calendar||1105 BH – 1104 BH|
|Minguo calendar||2362 before ROC
|Thai solar calendar||92–93|
−324 or −705 or −1477
— to —
−323 or −704 or −1476
Year 451 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Sabinus and Augurinus and the First year of the decemviri (or, less frequently, year 303 Ab urbe condita). The denomination 451 BC 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.
- The Persian fleet moves against a rebellious Cyprus to restore order. Kimon, who returns to favour, though not to power, in Athens, plans an expedition to help Cyprus. He gains Pericles' support.
- An Athenian law sponsored by Pericles is passed giving citizenship only to those born of Athenian parents. This marks an end to the policy where residents who were from other cities could be given an honourable status.
- Hostilities among the Greek states come to a formal end with the agreement to the Five Years' Truce. Kimon negotiates the five years' truce with Sparta, in which Athens agrees to abandon its alliance with Argos, while Sparta promises to give up its alliance with Thebes. During the same year Argos signs the first "Thirty-Years Peace" with Sparta.
- Following the report of a three-man commission into the design of Roman law, the patricians set up in Rome a Board of Ten, the Decemviri. The first Decemviri, composed entirely of patricians is led by consuls Appius Claudius Crassus and Titus Genucius Augurinus. The first ten codes of the Roman Law of the Twelve Tables are completed by the first Decemvirate.