This article studies the concept of “civil war” that Reinhart Koselleck used in his writings from his perspective as a historian of concepts and during a Seminar on the Spanish Civil War that he organized in 1961. We will review the main works published by Koselleck, but the main purpose of this study is the analysis of some unpublished work-notes that we have discovered in the Deutsches Literatur Archiv of the German city of Marbach. This unpublished documentation refers to notes Reinhart Koselleck compiled while preparing for the seminar on the Spanish Civil War that took place at the University of Heidelberg from March 5 to 10, 1961.