Portrait of Astley David Middleton Cooper at his easel. Paintings of Native Americans captivated Cooper. His grandfather, Major Benjamin O'Fallon, fought in the American Indian Wars, and his great great uncle, William Clark, partnered in the famous Lewis and Clark Expedition. However, his greatest influence was George Catlin, a nationally famous painter of Native Americans and a close family friend. Both Catlin and Cooper were trying in their paintings to preserve a way of life under threat from rapid Westward expansion. The Native Americans pictured in these paintings are likely Sioux.