What was the most important war in Texas history? The Civil War. In the U.S., during the 1850’s with the north and south, the Civil War happened with the texans. So, why did Texans fight? The causes of the Texans fighting were slavery, sectionalism, and state’s rights. To begin, state’s rights was one of the reasons on why Texans fought. For example, Abraham Lincoln said that “...no state upon its own mere motion, can lawfully get out of the union.”Doc.C. This shows that the states had no right to get out of the union, which was forbidding the state’s rights which texans certainly wanted. So, this caused the Civil War. Furthermore, sectionalism was also one of the causes of the Civil War. For instance, in document B, it says