Complex sentence: The main office of "The New York Times" was attacked during the New York Draft Riots sparked by the beginning of military conscription for the Northern Union Army now instituted in the midst of the Civil War on July 13,1863.
Split into simpler sentences: The main office of "The New York Times" was attacked during the New York Draft Riots. The riots, sparked by the beginning of drafting for the Union Army, beginning on July 13,1863.