Complex sentence: Extensive remodeling following World War II until the early 1950s led to expansion in the downtown Dallas store, which fostered the development of its band instrument division, which would become the largest supplier of band instruments to school groups in the southern United States.
Split into simpler sentences: Extensive remodeling following World War II until the early 1950s led to expansion in the downtown Dallas store, which fostered the development of its band instrument division. It became the largest supplier of band instruments to school groups in the southern United States.