The battle of Cos was the decisive battle of the Second Syrian War. It was fought between the fleets of Ptolemy II of Egypt and Antigonus of Macedonia, and marked a resurgence of Macedonian naval power. During the Chremonidean War an Egyptian fleet had virtually blockaded Macedonia, restricting Antigonus to mainland Greece.
Egypt was the principle naval power of the eastern Mediterranean. Ptolemy II could call on a fleet of 300 ships, the expertise of Phoenicia and was allied to Rhodes, the possessor of a small but powerful fleet. This fleet had played an important role in his victories during the First Syrian War.
Antigonus responded by building up a new war fleet. His main source of warships was Corinth, where the tradition was for larger warships, designed to board and capture enemy ships. Antigonus had a massive flagship built at Corinth, initially named Corinthia, and containing enough wood to build fifteen normal warships. In addition, Antigonus and his ally Antiochus II, the Seleucid Emperor, persuaded Rhodes to change sides and support them against Ptolemy. Even so, Antigonus was probably outnumbered by as much as two-to-one at Cos.
The battle of Cos is generally dated to 258 BC. It took place during the Isthmian Games, which took place every two years, giving three potential dates during the Second Syrian War – 260, 258 or 256. Of these the middle date seems most likely – a fleet from Rhodes is known to have defeated an Egyptian squadron close to Ephesus in 259 BC, while 256 is seen as coming too late in the war. Never-the-less, the Second Syrian War is not well documents, so any of these dates is possible.
The two fleets clashed off the island of Cos, in the south east Aegean. At first glance one might therefore thing that this battle took place at quite a distance from Ptolemy’s Egyptian kingdom, but at this time he also held a variety of possessions in and around the Aegean.
Very little detail is known of the battle. Antigonus commanded the Macedonian fleet in person, from his massive flagship. He was badly outnumbered, but still won a crushing victory. In the peace treaty that concluded the war (255 BC), Ptolemy was forced to surrender most of his Aegean possessions.
Antigonus celebrated his victory at Cos in several ways. He built a portico on Delos to mark his new domination of the sea. His flagship was renamed Isthmia, dedicated to the god Apollo and housed in a temple on Delos.
The loss of his fleet prevented Ptolemy from protecting his possessions in Asia Minor. By the time the war ended he had lost most of the land he had won in the First Syrian War.