The Ottoman Empire and Romania declared war on Bulgaria , because they wanted to gain Bulgarian territories , and by the end of the Second Balkan War Bulgaria had lost almost all its gains from the First Balkan War . Their armies repulsed the Bulgarian offensive and counter-attacked penetrating into Bulgaria , while Romania and the Ottoman Empire took the opportunity to intervene against Bulgaria and make territorial gains . In the resulting Treaty of Bucharest , Bulgaria lost most of the territories it had gained in the First Balkan War .