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|Millennium:||1st millennium BC|
|Centuries:||6th century BC – 5th century BC – 4th century BC|
|Decades:||440s BC 430s BC 420s BC – 410s BC – 400s BC 390s BC 380s BC|
|Years:||421 BC 420 BC 419 BC – 418 BC – 417 BC 416 BC 415 BC|
|418 BC by topic|
|Gregorian calendar||418 BC
|Ab urbe condita||336|
|Ancient Egypt era||XXVII dynasty, 108|
|- Pharaoh||Darius II of Persia, 6|
|Ancient Greek era||90th Olympiad, year 3|
|Chinese calendar||壬戌年 (Water Dog)
2279 or 2219
— to —
癸亥年 (Water Pig)
2280 or 2220
|Coptic calendar||−701 – −700|
|Ethiopian calendar||−425 – −424|
|- Vikram Samvat||−361 – −360|
|- Shaka Samvat||N/A|
|- Kali Yuga||2684–2685|
|Iranian calendar||1039 BP – 1038 BP|
|Islamic calendar||1071 BH – 1070 BH|
|Minguo calendar||2329 before ROC
|Thai solar calendar||125–126|
|Wikimedia Commons has media related to 418 BC.|
Year 418 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Fidenas, Axilla and Mugillanus (or, less frequently, year 336 Ab urbe condita). The denomination 418 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.
- King Agis II of Sparta escapes having his house razed and being fined 100,000 drachmae for his failure to press his advantage by promising more successful outcomes in the future.
- The Battle of Mantinea is the largest land battle of the Peloponnesian War (with as many as 10,000 troops on each side). Sparta under King Agis II has a major victory over Argos (and its allies Athens, Ellis and Mantinea), which has broken its treaty with Sparta's King Agis II at the insistence of Alcibiades. Agis II's major victory makes amends with the Spartans for his earlier truce with Argos. The commander of the Athenian forces, Laches, is killed in the battle.
- Impressed with the Spartan victory, the inhabitants of Argos change their government from democracy to oligarchy and end their support for Athens in favour of an alliance with Sparta. Many of Argos' allies do the same. Athens becomes increasingly isolated.
- Alcibiades urges the Athenians to conquer Syracuse, subdue Sicily and Carthage and thus gain added forces that will enable them to finish the war against Sparta. His bold offensive plan wins the support of the Athenians.
- Epaminondas of Thebes, Theban general and statesman (d. 362 BC)
- Iphicrates, Athenian general (approximate date) (d. c. 353 BC)