Pericles takes a fleet into the Black Sea at an unknown date, but probably in the early 430s, to demonstrate Athenian power and secure the grain route from the Tauric Chersonese (the Crimean Peninsula in modern Ukraine). (According to another view, this event belongs in the 450s.)

The Romans break a forty-year truce with Veii, the richest city of the Etruscan League, on the southern border of Etruria, by attacking nearby Fidenae, an important outpost of the city just five miles from Rome, thus initiating in 438 BCE what will prove to be a long war.

Cratinus, a celebrated Athenian writer of comedies whose subjects are political, religious, literary, and mythological, makes Pericles a favorite target of his inventive and exuberant satire.

Athens makes another large-scale attempt, this one in 437-436 BCE successful, at colonizing Ennea Hodoi, “Nine Roads”,.

Located on the Strymon (Strimón) River about three miles from the Aegean Sea, in Macedonia, the new city, Amphipolis, is a strategic transportation center, controlling the bridge over the Strymon and the route from northern Greece to the Hellespont, including the western approach to the timber, gold, and silver of Mount Pangaeum in Thrace.

The newest Athenian colony is disconcertingly close to another outpost of Corinthian influence at Potidaea in the Chalcidice, and there is a possibility that Athens subjects Potidaea itself to financial pressure by the mid-430s.

An anomaly in being both tributary to Athens and simultaneously subject to direct rule by magistrates sent out annually by Corinth, Potidaea clearly is a sensitive spot in international relations.

Corinth is being indirectly pressured by Athens to the west (Acarnania and other places) and northeast (Amphipolis, Potidaea); …

…this pressure is felt in Corinth's own backyard, at Megara.

Athens passes a series of measures (the “Megarian decrees”) imposing an economic embargo on Megara for violations of sacred land.

The religious aspect of the offense is reflected in the exclusions imposed: like murderers, the Megarians are banned from the Athenian marketplace and the harbors in the Athenian empire.

However, there is no doubt that Athens causes and intends to cause economic hardship as well or that the decrees are the first move in securing Megara as a military asset.

Corinth attempts to control the strategically important Corcyran colony of Epidamnus on the coast of Illyria (a colony in which Corinth also had an interest, heretofore controlled jointly with the island of Corcyra (modern Corfu, or Kerkira), provoking a naval war instigated by the latter.

As the Corcyran fleet of eighty triremes moves on Epidamnus in 435, a Corinthian fleet of seventy-five ships attempts to relieve the colony but suffers a serious defeat at the Gulf of Ambracia (Arta).

Philip, having enlisted the support of Athens and King Derdas of Elimea, is challenging his older brother Perdiccas for the Macedonian throne by 434 BCE.

Perdiccas responds by stirring up rebellion in a number of Athenian tribute cities, including Potidaea.

Athens responds with force, sending one thousand hoplites and thirty ships to Macedonia where they capture Therma.

They go on to besiege Pydna, where they are met by reinforcements of a further two thousand hoplites and forty ships.

However, as the Athenians are besieging Pydna, they receive news that Corinth has sent a force of sixteen hundred hoplites and four hundred light troops to support Potidaea.

In order to combat this new threat, Athens makes an alliance with Perdiccas, and proceeds to Potidaea.

Perdiccas immediately breaks the treaty and marches to Potidaea, while the Athenians are eventually victorious, the battle (along with the Battle of Sybota) directly leads to the Peloponnesian War.

Anaxagoras, a native of Clazomenae, had brought philosophy and the spirit of scientific inquiry from Ionia to Athens.

His observations of the celestial bodies have led him to form new theories of the universal order.

Anaxagoras describes the cosmos as a continuous field in which different qualities flow and mingle.

He suggests that are not four but possibly thousands of elements and that, as no element can become another, there is no real change but only the illusion of change manifested by the combination, separation and recombination of elements.

He attempts to give a scientific account of eclipses, meteors, rainbows and the sun, which he describes as a mass of blazing metal, larger than the Peloponnese.

The heavenly bodies, he asserts, are masses of stone torn from the earth and ignited by rapid rotation.

However, these theories bring him into collision with the popular faith; Anaxagoras' views on such things as heavenly bodies are considered "dangerous."

This new Ionian science is not well-received by the Athenian public, who view Anaxagoras’ teachings—that the divine mind in this system does not have a human body, and that the Luminaries are not gods but rocks—as deeply irreligious.

Anaxagoras is arrested by Pericles' political opponents on a charge of contravening the established dogmas of religion (some say the charge was one of Medism).

It takes Pericles' power of persuasion to secure his release.

Even so he is forced to retire from Athens to Lampsacus in Ionia in about 434-433 BCE.