Final answer:
The Helots were able to successfully revolt due to internal strife among the Spartans and Persian intervention.
Step-by-step explanation:
The Helots were able to successfully revolt due to a combination of factors, including internal strife among the Spartans and Persian intervention. The Spartans, who had maintained control over the Helots through fear and violence, were preoccupied with suppressing rebellion and had a small military force compared to the Helots. Additionally, Persian backing of the Spartans' enemies weakened their control over the Helots.