Year 433 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Vibulanus, Fidenas and Flaccinator (or, less frequently, year 321 Ab urbe condita). The denomination 433 BC for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.
Pericles concludes a defensive alliance with Corcyra (Corfu), the strong naval power in the Ionian Sea, which is the bitter enemy of Corinth. As a result, Athens intervenes in the dispute between Corinth and Corcyra, and, at the Battle of Sybota, a small contingent of Athenian ships play a critical role in preventing a Corinthian fleet from capturing Corcyra. Following this, Athens places Potidaea, a tributary ally of Athens but a colony of Corinth, under siege.
The Corinthians, upset by Athens' actions, lobby Sparta to take action against Athens. This appeal is backed by Megara (which is being severely affected by Pericles' economic sanctions) and by Aegina (which is being heavily taxed by Pericles and which has been refused home rule).
Pericles renews alliances with the Rhegium on the south west corner of Italy and Leontini in south-east Sicily, threatening Sparta's food supply route from Sicily.