What primary source document could help a historian understand why the US Civil War started?

A primary source document that could help a historian understand why the US Civil War started is the "Declaration of the Causes of Seceding States," particularly Texas's declaration. This document outlines the reasons Texas and other Southern states felt compelled to secede from the Union, emphasizing issues such as states' rights, economic interests, and the significance of slavery. Analyzing this declaration provides insights into the ideological and political tensions that contributed to the outbreak of the war.