that led to prolonged conflict. By the tenth year of this war the Spartans and Messenians had reached an agreement in which Messenian rebels were allowed to leave the Peloponnese. They were given safe passage under the terms that they would be re-enslaved if they tried to return. This agreement ended the most serious incursion into Spartan territory since their expansion in the seventh and eighth centuries BC. Thucydides remarked that "Spartan policy is always mainly governed by the necessity of taking precautions against the helots." On the other hand, the Spartans trusted their helots enough in 479 BC to take a force of 35,000 with them to Plataea, something they could not have risked if they feared the helots would attack them or run away. Slave revolts occurred elsewhere in the Greek world, and in 413 BC 20,000 Athenian slaves ran away to join the Spartan forces occupying Attica. What made Sparta's relations with her slave population unique was that the helots, precisely because they enjoyed privileges such as family and property, retained their identity as a conquered people (the Messenians) and also had effective kinship groups that could be used to organize rebellion. As the Spartiate population declined and the helot population continued to grow, the imbalance of power caused increasing tension. According to Myron of Priene of the middle 3rd century BC: Plutarch also states that Spartans treated the helots "harshly and cruelly": they compelled them to drink pure wine (which was considered dangerous – wine usually being cut with water) "...and to lead them in that condition into their public halls, that the children might see what a sight a drunken man is; they made them to dance low dances, and sing ridiculous songs..." during syssitia (obligatory banquets). Each year when the Ephors took office, they ritually declared war on the helots, allowing Spartans to kill them without risk of ritual pollution. This fight seems to have been carried out by kryptai (sing. κρύπτης kryptēs), graduates of the agoge who took part in the mysterious institution known as the Krypteia. Thucydides states: The helots were invited by a proclamation to pick out those of their number who claimed to have most distinguished themselves against the enemy, in order that they might receive their freedom; the object being to test them, as it was thought that the first to claim their freedom would be the most high