The phrase "was on the back of" typically implies a position of inferiority, behind or subordinate to another. Some antonyms to consider could be "was at the forefront of," "took the lead in," "led the charge," or "was in the vanguard of." These phrases suggest a leadership or pioneering role, positioning the subject as a leader or trailblazer rather than following from behind. Other antonyms might include "was beside," "was alongside," or "was equal to," indicating a position of parity rather than one of subservience. Depending on the context, there may be other antonyms that could be appropriate as well.