Complex sentence: Starting in the mid- 1880s a new group, the Knights of Labor, grew rapidly... too rapidly, for it spun out of control and failed to handle the Great Southwest Railroad Strike of 1886.
Split into simpler sentences: Starting in the mid- 1880s a new group, the Knights of Labor, grew rapidly. Too rapidly, for it spun out of control and failed to handle the Great Southwest Railroad Strike of 1886.