Complex sentence: During the American Civil War he was successfully involved in grain and cereal dealing, but by refusing to become involved in corrupt operations, his rivals were able to move against him and Tibbets lost his fortune.
Split into simpler sentences: During the American Civil War, he was successfully involved in grain and cereal dealing. By refusing to become involved in corrupt operations, he could not compete with rivals, who drew off his business, causing him to lose most of his fortune.