In 460 BC, Inaros II, a pharaoh from an ancient line of Egyptian rulers, rebelled against the Persian Empire, then the sovereign of Egypt. The Athenians decided to support the rebellion. Egypt was a prize worth fighting for because if the Egyptian rebellion succeeded, Athens would be assured of a regular supply of grains. In the same year, there was the outbreak of the First Peloponnesian War, in which the Athenian alliance was pitted against the Spartan alliance. The Athenian thinking at that time was that they possessed the military and economic resources to fight the Spartans and at the same time provide military aid to the Egyptian rebels.
In the early stages of the Egyptian rebellion, the momentum was on the Athenian and Egyptian side. They killed the Persian general Achaemenes in 460 BC, and forced the Persian army to retreat to Memphis, a city in Lower Egypt, where the Persians made their stand. The Athenians and the Egyptians besieged the city. The contest between the Persians and the Athenians was simultaneously happening on the sea. Athenian generals Charitimides and Cimon sank a number of Persian vessels and captured a few of them.
The primary theatre of war for the Athenians was not Egypt—it was the Greek world, where they were pitted against the Spartan alliance. Here too they were doing well. The Spartans had a string of mishaps in the land and sea battles, and towards the middle of the 450s BC, it appeared as if the Athenians might win the First Peloponnesian War. Being confident of victory, they pulled out some naval resources from the Peloponnesian War and deployed them in the Egyptian rebellion. But they had underestimated the resolve of Persian Emperor Artaxerxes, son of Xerxes, to mount a counteroffensive to save the Egyptian part of his empire.
Artaxerxes dispatched reinforcements with a new general called Megabyzus to Egypt. The arrival of Megabyzus changed the situation in Memphis which was being besieged by the Athenians and the Egyptians for more than four years. He adopted an aggressive strategy. The Persian army poured out of the city while the reinforcements that he had brought with him from Persia attacked the besieging army from the other side. The Athenians and the Egyptians were caught between two waves of Persian armies and there was chaos in their ranks. They were forced to retreat.
The Egyptian rebels melted into the countryside, and while a small group of Athenian soldiers retreated to an island called Prosopitis in the Nile Delta where their triremes were moored. But before they could launch their triremes into the sea, Megabyzus arrived with his army and surrounded the island. During the eighteen months of standoff that followed, the Persians drained the Nile Delta by digging several canals. With all water around the island gone, the Athenian triremes were now stuck on dry land. The Persians walked over the dry river bed and captured the triremes and the Athenian soldiers. A few Athenian soldiers managed to reach Athens after months of marching through Libya to Cyrene.
The Egyptian adventure was a major military disaster for the Athenians. They had lost not just a significant part of their army and naval vessels but also their reputation of naval superiority. The word spread through the Greek world that the entire Athenian navy was destroyed by the Persians. A series of rebellions erupted in the Athenian Delian league. This was a decisive point in the power struggle within the Greek world, as the balance of power started shifting towards the Spartan side. The Athenian leadership realized that they had invested too much resources in Egypt and they brought an end to the commitments that they had made to the rebels led by Inaros II. In 454 BC, Inaros II was captured by the Persian forces and executed.
In 451 BC, the Athenian leader Pericles recalled Cimon, who was ostracized and exiled in 461 BC, and ordered him to use his good connections with the Spartans for negotiating a five year temporary peace with them. Cimon succeeded in negotiating a temporary peace. Now Pericles was free to deploy the full strength of the Athenian navy on settling the conflict with the Persians. In 449 BC, the Athenian navy defeated the Persian fleet near Callias. This defeat forced the Persians to come to the negotiating table. The Peace of Callias was negotiated and the war between the Athenians and the Persians came to an end.
The next step for Pericles was to negotiate with the Spartan King Pleistoanax. They agreed to thirty years of peace between Athens and Sparta (starting from 446 BC). In the statement of account that Pericles later submitted to the Athenian assembly, he had mentioned "10 talents necessary expenses.” This fueled the rumor in the Greek world that Pericles had given 10 talents to Pleistoanax as a bribe to get him to agree to the Thirty Years’ peace treaty. The Spartans believed the rumors and they exiled Pleistoanax, but they did not rescind the peace treaty.