Complex sentence: John F. Rague is credited with designing the Territorial Capitol Building, he had previously designed the 1837 capitol of Illinois and was supervising its construction when he got the commission to design the new Iowa capitol in 1839.
Split into simpler sentences: John F. Rague is credited with designing the Territorial Capitol Building. He had previously designed the 1837 capitol of Illinois and was supervising its construction when he got the commission to design the new Iowa capitol in 1839.