The phrase "were mercy of" could be replaced with a number of synonyms depending on the context of the sentence. Some options include "at the mercy of," "subject to," "vulnerable to," "exposed to," "helpless against," "under the control of," and "dependent on." Each of these phrases implies a lack of control or power in a given situation, and suggests that the subject is vulnerable to outside forces. It is important to carefully consider which synonym to use in order to accurately convey the intended meaning of the sentence.