As Aratus was patriotic, he was anxious to finish the work which his father had begun. At the age of
twenty, therefore, he assembled a few comrades, entered Sicyon, called all the lovers of liberty to his aid, and drove away the tyrant without shedding any blood.
The town, thus freed, joined the Achæan League, of which Aratus soon became the leader. This office was elective, and no one was expected to fill it for more than a year; but Aratus was so much loved that he was chosen leader thirty-five years in succession.
At this time, Greece and Macedon were under the rule of Antigonus Go-na´tas, son of Demetrius; for this man had conquered for himself the second kingdom which his father had lost. But now Aratus and the Achæan 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 Ac-ro-co-rin´thus, 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, Di´o-cles, however, offered to show Aratus a way to climb this rock, provided that he should receive a certain reward. Although general of the Achæan 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 Achæan 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 Achæans gained such victories, that Antigonus Gonatus fell ill, and died of grief.