http://www.idbsu.edu/courses/hy101/peloponn/05.htmThe Peloponnesian War - Page 5 of 17
In 432, land forces engaged over Megara, which is on the isthmus
of Corinth. Then, in 431, Thebes attacked Plataea in an attempt to force
that city to join its own
Boeotian League. Plataea, you should recall, was the one city to fight
alongside Athens at Marathon. The Athenians still offered special thanks
to the city every five years in remembrance. There was no way Athens would
tolerate Theban domination of Plataea. Athens declared war on Thebes, Sparta
sprang to the defense of its ally, and everybody chose up sides, and the
war was on.