Complex sentence: One of the first times when incendiary bullets were used was in World War I. At the time, phosphorus--the primary ingredient in the incendiary charge--burned upon firing, leaving a trail of blue smoke.
Split into simpler sentences: One of the first times when incendiary bullets were used was in World War I. At the time, phosphorus was the primary ingredient in the incendiary charge. It burned upon firing, leaving a trail of blue smoke.