When did Austria - Hungary annex the de jure Ottoman province of Bosnia and Herzegovina ?
The rebels were aided with weapons and volunteers from the Principalities of Montenegro and Serbia , whose governments eventually jointly declared war on the Ottomans on 18 June 1876 , leading to the Serbo - Turkish War ( 1876–78 ) and Montenegrin – Ottoman War ( 1876–78 ) , which in turn led to the Russo - Turkish War ( 1877–78 ) and Great Eastern Crisis .