Simplify the following text for a Grade 11 native English-speaking person.
Original: Though the Balkan allies had fought together against the common enemy , that was not enough to overcome their mutual rivalries . The Second Balkan War broke out on 16 June 1913 when Bulgaria attacked its erstwhile allies in the First Balkan War , Serbia and Greece , while Montenegro , Romania and the Ottoman Empire intervened later against Bulgaria .
Simplified: The First Balkan War was fought by Greece , Serbia , Montenegro and Bulgaria ( the Balkan League ) against the Ottoman Empire .