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|Millennium:||1st millennium BC|
|387 BC by topic|
|Gregorian calendar||387 BC|
|Ab urbe condita||367|
|Ancient Egypt era||XXIX dynasty, 12|
|- Pharaoh||Hakor, 7|
|Ancient Greek era||98th Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||癸巳年 (Water Snake)|
2310 or 2250
— to —
甲午年 (Wood Horse)
2311 or 2251
|Coptic calendar||−670 – −669|
|Ethiopian calendar||−394 – −393|
|- Vikram Samvat||−330 – −329|
|- Shaka Samvat||N/A|
|- Kali Yuga||2714–2715|
|Iranian calendar||1008 BP – 1007 BP|
|Islamic calendar||1039 BH – 1038 BH|
|Minguo calendar||2298 before ROC|
|Thai solar calendar||156–157|
−260 or −641 or −1413
— to —
−259 or −640 or −1412
Year 387 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Papirius, Fidenas, Mamercinus, Lanatus and Poplicola (or, less frequently, year 367 Ab urbe condita). The denomination 387 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.
- Antalcidas, commander of the Spartan navy, actively assists Persia against Athens. After escaping from the Athenian blockade at Abydos, he attacks and defeats a small Athenian force, then joins his fleet with a supporting fleet sent from Syracuse. With this force, which is soon further augmented with ships supplied by the Persian satraps of the region, he sails to the Hellespont, where he is in a position to cut off the trade routes that bring grain to Athens.
- The Persians, unnerved by some of Athens' actions, including supporting King Evagoras of Cyprus and Pharaoh Hakor of Egypt (both of whom are at war with Persia), decide that their policy of weakening Sparta by supporting its enemies is no longer wise. So Antalcidas enters into negotiations with the Persian satrap Tiribazus and reaches an agreement under which the Persians will enter into the war on the Spartan side if the allies refuse to make peace.
- With Antalcidas' Spartan fleet in control of the Hellespont, Sparta deprives Athens of her Bosporus trade and tolls. The Athenians, mindful of being in a similarly serious situation as when defeated in the Peloponnesian War less than two decades before and facing Persian intervention on Sparta's side, are thereby ready to make peace.
- With the support of the Persian King Artaxerxes II, King Agesilaus II of Sparta concludes "the King's Peace" (or the Peace of Antalcidas, after the Spartan envoy and commander) with Greek allied forces in a manner favourable to Sparta. Under the Peace, all the Asiatic mainland and Cyprus remain under Persian control, Lemnos, Imbros, and Scyros remain Athenian dependencies, and all the other Greek states are to receive autonomy. By the King's Peace, the Persians become key players in Greek politics.
- Under the threat of Spartan intervention, Thebes disbands its league, and Argos and Corinth end their shared government. Corinth, deprived of its strong ally, is incorporated back into Sparta's Peloponnesian League. After eight years of fighting, the Corinthian War is at an end.
- Plato founded the Platonic Academy in Athens, where he taught Aristotle until 347 BC.
- With the aid of the Lucanians, Dionysius I of Syracuse devastates the territories of Thurii, Crotone, and Locri in mainland Italy. When Rhegium falls, Dionysius becomes the chief power in Greek Southern Italy. He then turns his attention to the Adriatic.
- Plato is forced by Dionysius to leave Syracuse after having exercised the right of free speech too broadly. Plato returns to Athens, outside which he founds a school.
- Rome begins to rebuild after being invaded by the Gauls under Brennus.
- Marcus Furius Camillus introduces the Capitoline Games (Ludi Capitolini) in honour of Jupiter Capitolinus, and in commemoration of Rome's Capitol not being captured by the Gauls.