The Thirty Years' Peace was a treaty, signed between the ancient Greek

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 city-states Athens

Athens

Athens , is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, as its recorded history spans around 3,400 years. Classical Athens was a powerful city-state...

 and Sparta

Sparta

Sparta or Lacedaemon, was a prominent city-state in ancient Greece, situated on the banks of the River Eurotas in Laconia, in south-eastern Peloponnese. It emerged as a political entity around the 10th century BC, when the invading Dorians subjugated the local, non-Dorian population. From c...

, in the year 446

446 BC

Year 446 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Barbatus and Fusus...

/445 BC

445 BC

Year 445 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Augurinus and Philo...

. The treaty brought an end to the conflict commonly known as the First Peloponnesian War

First Peloponnesian War

The First Peloponnesian War was fought between Sparta as the leaders of the Peloponnesian League and Sparta's other allies, most notably Thebes, and the Delian League led by Athens with support from Argos. This war consisted of a series of conflicts and minor wars, such as the Second Sacred War...

, which had been raging since c.460 BC.

Athens was forced to give up its conquests of Nisaea, Pegae, Troizan, and Achaea. This also ruled out armed conflict between Sparta and Athens if at least one of the two wanted arbitration. Neutral poleis could join either side (i.e. Sparta or Athens) and this implies that there was a formalized list of allies for each side. Athens and Sparta would keep all other territories pending arbitration. It also recognized both Leagues as legitimate, a boost for Athens and its newly formed empire in the Aegean.

The Thirty Years' Peace, however, only lasted 13 years: for in the year 432, Athens attacked Potidaea

Potidaea

Potidaea was a colony founded by the Corinthians around 600 BC in the narrowest point of the peninsula of Pallene, the westernmost of three peninsulas at the southern end of Chalcidice in northern Greece....

, which was a listed ally but a Corinthian colony. Corinth supported its former colony. Sparta supported Corinth, who had been engaged in battle with the Athenians in Corcyra in 435 BC

435 BC

Year 435 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the First year of the Consulship of Iullus and Tricostus...

. The Potidaean affair and Athenian trade sanctions against the Spartan ally Megara

Megara

Megara is an ancient city in Attica, Greece. It lies in the northern section of the Isthmus of Corinth opposite the island of Salamis, which belonged to Megara in archaic times, before being taken by Athens. Megara was one of the four districts of Attica, embodied in the four mythic sons of King...

 prompted the Spartans to declare that the Athenians had violated the treaty, thus declaring war. At this point the Thirty Years' Peace was void and the second Peloponnesian War (commonly known as the Peloponnesian War

Peloponnesian War

The Peloponnesian War, 431 to 404 BC, was an ancient Greek war fought by Athens and its empire against the Peloponnesian League led by Sparta. Historians have traditionally divided the war into three phases...

) began.