The Peace of Nicias (421 BC) brought a temporary end to the fighting in the Great Peloponnesian War. Although it was meant to last for fifty years, it was broken after only a year and a half, and the war continued until 404 BC.
The first tentative peace negotiations began after the Spartan defeat on Sphacteria in 425 BC. This battle ended with the surrender of 292 hoplites trapped on the island, amongst them 120 full Spartans. One weakness of the Spartan system was that there were very few full Spartans, and so the loss of 120 of them would have been a disaster. At first the peace negotiations had been unsuccessful, but in 424 BC the Athenians suffered a defeat at Delium, while the Spartan commander Brasidas was winning victories in Thrace and threatening Athens's grain supply. In 423 the two sides agreed to a one-year long truce. After this expired in 422 the Athenians send an expedition to Thrace, under the command of Cleon. This ended in another military disaster, at Amphipolis. Cleon and Brasidas were both killed in this battle, and with them went two of the main obstacles to peace.
Both sides now had good reasons to desire peace. The Athenians had suffered two costly defeats in a row and were not as confident as before. They were also worried that some of their allies might revolt if the run of defeats continued.
The Spartans were still worried about the men captured on Sphacteria. Their territory was being raided, and they were worried that this might inspire a helot uprising. They were also faced with the prospect of a war on two fronts. The thirty-year truce between Sparta and their Peloponnesian neighbour and enemies Argos was about to expire, and was unlikely to be renewed.
Thucydides also picked out one political leader on each side who had a particular desire for peace, and assigned them somewhat unflattering motives. Nicias, son of Niceratus, one of the more successful Athenian commanders, was said to be motivated by a desire to end his active military career without any disasters. King Pleistoanax of Sparta, who had been exiled for nineteen years before being restored as king was said to have wanted to distract attention from the nature of his restoration, and hoped that the return of Spartan prisoners would achieve that.
Thucydides records eighteen main clauses of the peace treaty.
1 - Temples and those travelling to them were to be respected.
2 - Delphi was to be self-governing.
3 - The treaty was to last fifty years
4 - It was to be illegal to fight for the Spartans against the Athenians or for the Athenians against the Spartans (and both sets of allies).
5 - Disputes were to be solved by law and by oath
6 - Amphipolis was to be returned to Athens.
7 - The citizens of cities being returned to Athens were to be allowed to move wherever they want, with their property
8 - The cities returned to Athens were to be independent, and were to pay the tribute fixed by Aristides
9 - The Athenians were not allowed to take up arms against any city that paid its tribute.
10 - The cities included in clauses seven, eight and nine were listed as Argilus, Stagirus, Acanthus, Scolus, Olynths and Spartolus.
11 - The Mecybernaeans, Sanaeans, Singaeans, Olynthians and Acanthians were allowed to inhabit their own cities.
12 - Sparta shall return Panactum to Athens
13 - Athens shall return Coryphasium, Cythera, Methana, Ptelium and Atalanta to the Spartans and also to return all Spartan prisoners in Athens or in the Athenian dominions.
14 - All Peloponnesians and allies of Sparta trapped within the besieged city of Scione are to be allowed to leave. The siege itself continued, and ended with a massacre of the men of military age.
15 - The Spartans shall return any Athenian or allied prisoners.
16 - The Athenians were free to do whatever they want with Scione, Torone, Sermyle and any other cities in their hands.
17 - Athens and their allied cities and Sparta and their allied cities were to swear an annual oath to obey the treaty.
18 - Any point that had been overlooked could be added to the treaty by agreement between Sparta and Athens.
Effectively the treat restored the situation before the war began, although Athens was able to keep Nicaea and Sparta the city of Plataea, both places have changed sides after an agreement was made with the inhabitants.
This treaty was not popular with some of Sparta's allies. Amphipolis was unwilling to be returned to Athenian rule, and several others refused to sign without modifications. Amazing Sparta's response was to sign an alliance with Athens! Both cities agreed to come to the others aid if they were invaded and Athens agreed to support the Spartans against any slave uprising.
The peace of Nicias was never entirely effective, and was never entirely put into effect. Thucydides records it as lasting for six years and ten months, but this only refers to the period in which neither side directly invaded the others territory. In reality the war resumed after a gap of about a year and a half, and in 418 BC Athenians and Spartans once against faced each other on the battlefield, at Mantinea.