Year 429 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Tricipitinus and Fidenas (or, less frequently, year 325 Ab urbe condita). The denomination 429 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.
The Athenians under Xenophon march into Thrace to attack Chalcis. They destroy crops outside Spartolus and begin negotiating with pro-Athenian factions in Chalcis, but the anti-Athenian factions ask for help from Olynthus. An army from Chalcis, Spartolus, and Olynthus meet the Athenians in battle, but their hoplites are defeated. Reinforcements soon arrive from Olynthus, and they launch a second attack on the Athenians. The Athenians are routed, with all of their generals and 430 other men killed.
The Macedonian king, Perdiccas II, once again betrays the Athenians and sends 1000 troops to support a Spartan assault on Acarnania but they arrive too late to help. In response to this, King Sitalkes of Thrace invades Macedonia with a vast army that includes independent Thracian tribes (such as the Dii) and Paionian tribes (Agrianes and Laeaeans). His progress is slowed when the promised support from Athens fails to materialise. So Perdiccas once again uses diplomacy to ensure the survival of Macedonia. He promises the hand of his sister in marriage to the nephew of Sitalkes, who then persuades Sitalkes to leave Macedonia.
The plague in Athens that is killing thousands of the city's inhabitants, claims Pericles. Cleon, who has headed the opposition to Pericles' rule, succeeds to power in Athens following Pericles' death.