Complex sentence: On Orthodox Christmas, January 7,1919, Montenegrins staged a national uprising against the decision of the unconstitutional "Grand National Assembly" (Podgoricka skupstina) for annexation by Serbia.
Split into simpler sentences: On Orthodox Christmas, January 7,1919, a national uprising occurred in Montenegro. The catalyst was the decision of the unconstitutional "Grand National Assembly" (Podgoricka skupstina) for annexation by Serbia.