At this time, Greece and Macedon were under the rule of Antigonus Gonatas, son of Demetrius; for this man had conquered for himself the second kingdom which his father had lost. But now Aratus and the Achaean League refused to obey him, so he marched down from Macedon to restore order.

To prevent his advance, and to hinder his getting even as much as a foothold in the peninsula, Aratus wanted to capture the fortress of Acrocorinthus, which barred the Isthmus of Corinth.

This undertaking was very difficult, because the fortress was perched upon a rock so high and steep that it was almost impossible to climb it.

A traitor, Diocles, however, offered to show Aratus a way to climb this rock, provided that he should receive a certain reward. Although general of the Achaean League, and one of the greatest men of his day, Aratus was far from being rich; and, in order to obtain the required sum, he had to sell all he had, and even pawn his wife’s few jewels.

Then, in the midst of the darkness, one rainy night, Diocles led the Achaean soldiers along a steep path, which they had to climb in Indian file.

He brought them safely and unseen into the fortress, where they killed most of the Macedonian sentinels, and put the guards to flight. As soon as the key of the Peloponnesus had been thus daringly won, most of the other towns in the peninsula joined the league, and the Achaeans gained such victories, that Antigonus Gonatus fell ill, and died of grief.

The Achaean League became stronger and stronger; and, although Sparta and a few other cities remained neutral, most of the small towns were freed from their tyrants. Such was the importance of the league, that the Roman ambassadors once came to ask for its aid to suppress the pirates who infested the neighboring seas.