My thesis explores the relations between the Chickasaw and Choctaw Nations from 1839 to 1856. In 1839, the majority of Chickasaws were living in Indian Territory under the Choctaw Nation tribal government and in 1856, the Chickasaw Nation adopted their first constitution as an independent sovereign nation. It is an under researched time period with no secondary sources exclusively focused on it. When it is mentioned in secondary sources, it is often about one tribe or the other, not both, and they focus on two main arguments: The Chickasaw Nation never wanted to surrender their sovereignty, much less have to answer to the Choctaw Nation, and that the Choctaw Nation was unwilling to sell part of their new home in Indian Territory to the Chickasaw. However, research into primary sources written during the time period by Chickasaw and Choctaw leadership reveal a much more complicated answer that exposes the true motivations of each nation. I argue that the Chickasaws saw their inclusion in the Choctaw government as a temporary situation from the signing of the Treaty of Doaksville in 1837 and the Choctaw Nation only agreed to the Treaty of 1856 in order to hold the U.S. accountable for outstanding claims. I consider the long history between the Chickasaw and Choctaw people, which includes shared origin and migration stories and the blood wars of the 1700s that provides further context for their relationship. Using a Chickasaw historical research methodology, a Chickasaw lens, and traditional historical research methods to analyze historical documents, letters, and tribal and U.S. government records, I reveal the complex, sibling-like relationship of the Chickasaw and Choctaw Nations leading up to their historical separation in 1856.