These, like much other international law (see law, international), depended on custom and showed a constant conflict between the higher standards of optimistic theory and the harsher measures permitted by actual usage, while passion and expediency frequently caused the most fundamental rules to be violated. Thus, the temptation to profit from a surprise at times led to the opening of hostilities without a declaration of war. Probably the law most generally observed was that of the sanctity of *heralds, for heralds were essential to communications between belligerents. Nor did Greeks frequently refuse a defeated army a truce for burying its dead, for the request of such a truce meant an admission of defeat and was usually followed by retreat. Beyond this there were few restraints except humanitarian considerations and the universal condemnation of excessive harshness: Thucydides’ indignation at the massacre at Boeotian Mycalessus seems partly prompted by the victims’ status as (Greek) non-combatants: 7. 29–30. Plundering and the destruction of crops and property were legitimate, and were carried on both by regular armies and fleets, and by informal raiding-parties and privateers, and even the sanctity of temples was not always respected. Prisoners, if not protected by special terms of surrender, were at the mercy of their captors, who could execute them or sell them into *slavery (see booty).