Immediate causes of the war
The outbreak of the war came when the Spartans issued ultimatums to Athens that the
men of the Athenian assembly rejected at the urging of Pericles. The
Spartan
ultimatums promised attack unless Athens lifted its economic sanctions against the
city-state of Megara,
1 a Spartan ally that lay just
west of Athenian territory, and stopped its
military blockage of Potidaea,
2 a
strategically located city-state in northern Greece. The Athenians had forbidden the
Megarians from trading in all the harbors of the Athenian empire, a severe blow for
Megara, which derived much income from trade. The Athenians had imposed the
sanctions in retaliation for alleged Megarian encroachment on sacred land along the
border between the territory of Megara and Athens. As for Potidaea, it been an ally
of Athens but was now in rebellion. Potidaea retained ties to
Corinth3, the city that had originally founded it, and Corinth, an
ally of Sparta, had protested the Athenian blockade of its erstwhile colony. The
Corinthians were already angry at the Athenians for having supported the city-state
of
Corcyra4 in its earlier quarrel with
Corinth and securing an alliance with Corcyra and its formidable navy. The Spartans
issued the ultimatums in order to placate the Megarians and, more importantly, the
Corinthians with their powerful naval force.
Corinth had threatened to
withdraw from the Peloponnesian League5 and join a different international alliance if the Spartans delayed any
longer in backing them in their dispute with the Athenians over Potidaea. In this
way, the actions of lesser powers nudged the two great powers, Athens and Sparta,
over the brink to war in 431 B.C.
