The King's Peace or Peace of Antalcidas (Spring 386 BC) ended the Corinthian War (395-386 BC) and temporarily secured Spartan dominance of mainland Greece while at the same time acknowledging Persian control of the Greek cities of Asia Minor.
For most of the Corinthian War the Persians had been involved in a parallel war with Sparta (Persian-Spartan War, 400-387 BC), but by the mid 380s the Athenians had begun to alienate Artaxerxes II, partly by supporting revolts against his authority and partly by appearing to be attempting to restore their empire. By 388 Artaxerxes was ready to change sides. The pro-Athenian satrap of Sardis was replaced by his predecessor Tiribazus, originally replaced for suggesting peace with Sparta, and the pro-Athenian satrap Pharnabazus was replaced by Ariobarzanes, a friend of the Spartan diplomat Antalcidas. In response the Spartans appointed Antalcidas as the navarch for 388-387, and he and Tiribazus journeyed to Susa to meet with Artaxerxes.
They returned to Asia Minor in 387 with new peace terms. Artaxerxes claimed rule over the Greek cities of Asia Minor and the islands of Clazomenae and Cyprus, while all other Greek cities were to be autonomous. The only exceptions were to be Lemnos, Imbros and Scyros, which were recognised as belonging to Athens.
By this point all of the Greek powers were ready for peace, and in the spring of 386 they all signed the new peace treaty. The exact terms of the peace aren't clear - the basic outline of the proposal of 387 clearly remained, but later versions of the peace included extra clauses that might not have been in the initial treaty.
Its main victims were Thebes, which lost control of the Boeotian League and Argos, which had merged with Corinth during the war. Thebes attempted to bypass the treaty by signing the treaty on behalf of the Boeotian League. The Spartans mobilised their army, but the Thebans gave in before any fighting began. Argos and Corinth also attempted to save their union, but the Spartans threatened to invade, and the Argive garrison evacuated Corinth. The exiled pro-Spartan party was restored to power in Corinth, and the leaders of the anti-Spartan party fled to Athens.
The only exception to the new rule of autonomy was Sparta's own Peloponnesian League. This left Sparta as the dominant power in Greece, but backed up by Persian power, an uncomfortable position. In the years after the peace the Spartans used their position to attack a number of their enemies. The city of Mantinea in Arcadia was actually forced to dissolve by the Spartans and its inhabitants move back to their original twelve villages. However the Spartans overreached themselves in 382 BC when they occupied Thebes and imposed a pro-Spartan oligarchy in power. This soon triggered a Theban-Spartan War, which ended with the crushing Spartan defeat at Leuctra (371 BC), the first major defeat suffered by a full sized Spartan hoplite army, and a key step in the decline of the city.