Complex sentence: Corinth attempted to mediate the dispute, and achieved an agreement which set the borders between Thebes and Plataea, and a promise that Thebes would not interfere with cities which did not want to be a part of a Boeotian state.
Split into simpler sentences: Corinth attempted to mediate the dispute, and achieved an agreement that set the borders between Thebes and Plataea. In addition to this, Thebes made a commitment not to interfere with cities that did not want to be a part of a Boeotian state.