Complex sentence: After Romania declared war on Austro-Hungary on 22 August 1916, Mackensen was given command of a multi-national army of Bulgarians, Ottomans, Austro-Hungarians and Germans which assembled in northern Bulgaria and then advanced into southern Romania.
Split into simpler sentences: After Romania declared war on Austro-Hungary on 22 August 1916, Mackensen was given command of a multi-national army of Bulgarians, Ottomans, Austro-Hungarians and Germans. It assembled in northern Bulgaria and then advanced into southern Romania.