Civil war is an essentially contested concept, and has been since its Roman invention in the 1st century BCE. This lecture treats the confusions over the meaning and application of the term "civil war" in the era of the US Civil War (as it came to be called mostly after the conflict itself), with special reference to its legal redefinition in the Lieber Code (1863) amid contemporary discussions by Anna Ella Carroll, Victor Hugo, Abraham Lincoln, Karl Marx, Herman Melville and J. S. Mill.