Complex sentence: The St. Louis Soccer League was a soccer league based in St. Louis, Missouri, which existed from 1907 to 1938 and was at its founding the only fully professional soccer league in the United States.
Split into simpler sentences: The St. Louis Soccer League was a soccer league based in St. Louis, Missouri, that existed from 1907 to 1938. At its founding, it was the only fully professional soccer league in the United States.