Complex sentence: In winter of 1915 Army of Montenegro has been fighting Austro-Hungary for three months, resisting the invasion to their terriitory.Montenegrin Army was weakened by the harsh weather and lack of supplies.
Split into simpler sentences: In winter of 1915, the Army of Montenegro had been fighting Austro-Hungary for three months, resisting the invasion to their terriitory. TheMontenegrin Army was weakened by the harsh weather and lack of supplies.