The siege of Chalcedon (408 BC) was part of an Athenian attempt to regain control of the Bosphorus and ensure the safety of Athens's food supplies from the Black Sea (Great Peloponnesian War).
Chalcedon, on the Asian shore, and Byzantium, on the European shore, had been part of the Athenian Empire, but both cities rebelled after the Athenian defeat at Syracuse. The Athenians managed to regain control over the Hellespont at the battle of Cyzicus (410 BC), but had to wait another two years before they were in a position to move against Chalcedon and Byzantium.
The Chalcedonians were expected an attack in 408, and took the precaution of sending their movable wealth to apparent safety with their neighbours in Bithynia. The city was defended by the Chalcedonians and by a Spartan force, and the defence was commanded by the Spartan 'harmost' Hippocrates. A Persian army under the satrap Pharnabazus was also in the area.
The Athenian force had three commanders - Theramenes, Thrasyllus and Alcibiades, and contained at least 5,000 men and 70 ships. Alcibiades was probably in overall command, and it was him who led a force of cavalry and heavy infantry into Bithynia, where he seized the Chalcedonian's property.
After this expedition the Athenians constructed lines of circumvallation around Chalcedon, blockading the city in one direction and protecting against the Persians in the other. Hippocrates and Pharnabazus decided to try and break the siege with simultaneous attacks. Hippocrates emerged from the city, and offered battle. The Athenians accepted, and Thrasylus led their heavy infantry in an attack on the Chalcedonians and Spartans. The battle was undecided until Alcibiades attacked with a mixed force of infantry and cavalry. Hippocrates was killed, and his army broke and fled back into the city. Pharnabazus, who had been unable to break through the Athenian siege lines, retreated back to his camp.
After this battle Alcibiades left the Athenian army to raise more money. This left Theramenes in command, and he soon came to an agreement with Pharnabazus. He agreed to pat Athens twenty talents, while Chalcedon agreed to pay her normal tribute to Athens. He also granted the Athenians a safe conduct so that their ambassadors could visit the Persian emperor. In return the Athenians would stop all hostilities against Chalcedon. On his return Alcibiades was forced to take the same oath, although he only agreed to do so after Pharnabazus agreed to exchange oaths of good faith with him.
The Athenian embassy was a failure. The Spartans had already come to an agreement with the Persian emperor, and the Athenian ambassadors were detailed in Persian Asia Minor for three years before finally being released. In the meantime the Athenian army in the bosphorus moved on to attack Byzantium, which was betrayed to them after a siege.