Complex sentence: The defenders were a cross-section of Vukovar society; as many as a third were non-Croats, including Serbs, Ruthenians, Hungarians and other ethnicities.
Split into simpler sentences: The defenders were a cross-section of Vukovar society. As many as one-third were non-Croats, including Serbs, Ruthenians, Hungarians and members of other ethnicities.