The Battle of Potidaea was, with the Battle of Sybota, one of the catalysts for the Peloponnesian War. It was fought near Potidaea in 432 BC between Athens and a combined army from Corinth and Potidaea, along with their various allies.
Potidaea was a colony of Corinth on the Chalcidice peninsula, but was a member of the Delian League and paid tribute to Athens. After Sybota, Athens demanded that Potidaea pull down part of its walls, expel Corinthian ambassadors, and send hostages to Athens. Athens was afraid that Potidaea would revolt due to Corinthian or Macedonian influence, as Perdiccas II of Macedon was encouraging revolts among Athens' other allies in Thrace.
Athens gathered a fleet of thirty ships and 1000 hoplites under the overall command of Archestratus, which was originally meant to fight Perdiccas in Macedonia but was diverted to Potidaea. The Potidaeans sent ambassadors to Athens and Sparta, and when negotiations broke down in Athens, Sparta promised to help Potidaea revolt. The Athenian fleet sailed for Potidaea, but when it arrived, Archestratus attacked the Macedonians instead, as the Potidaeans had already revolted and allied with Perdiccas. Corinth sent 1600 hoplites and 400 light troops to Potidaea as well, under the command of Aristeus. In response, Athens sent out another 2000 hoplites and 40 more ships, under the command of Callias. After some fighting against Perdiccas, the combined Athenian forces sailed to Potidaea and landed there. Perdiccas and 200 of his cavalry joined with Aristeus, and their combined army marched to Potidaea as well.
In the ensuing battle, Aristeus' wing of Corinthian troops defeated a section of the Athenian line, but elsewhere the Athenians were victorious. Aristeus returned to Potidaea along the seacoast with some difficulty, hoping to avoid the main Athenian army. A reserve force of Potidaeans, located in nearby Olynthus, attempted to relieve Aristeus, but they were defeated as well. The Corinthians and Potidaeans lost about 300 men, and the Athenians about 150, including Callias. The Macedonian cavalry did not join the battle.
The Athenians remained outside Potidaea for some time, and were reinforced by another 1600 hoplites under the command of Phormio. Both sides built walls and counter-walls, and the Athenians succeeded in cutting off Potidaea from the sea with a naval blockade. During the blockade, representatives from Athens and Sparta met in Sparta, resulting in a formal declaration of war.
However, this siege seriously depleted the Athenian treasury, dumping as much as 1000 talents/year into this attack. This made the Athenian people unhappy, and in combination with the plague that swept through Athens in the early 420s BC, made the control of Pericles untenable. The Periclean strategy of hiding behind the Long Walls and relying on the low cash reserves of the Peloponnesians was starting to become unfavorable to the greater Athenian consciousness.
Alcibiades and Socrates were among the Athenian soldiers in this battle; Socrates saved Alcibiades life there.