Complex sentence: Ragen's Colts was a chiefly Irish street gang which dominated the Chicago underworld during the early twentieth century and by the late 1920s and 1930s became part of the Chicago Outfit under Al Capone.
Split into simpler sentences: Ragen's Colts was a chiefly Irish street gang which dominated the Chicago underworld during the early twentieth century. By the late 1920s and 1930s became part of the Chicago Outfit under Al Capone.