Who is the antagonist in The Fate of Liberty by Mark E. Neely, Jr.?

There isn't an antagonist in a fictional sense. The Fate of Liberty examines how Abraham Lincoln, his administration, and the military understand, practice, and justify the suspension of the writ of habeas corpus and imposition of martial law during the Civil War. It concludes the practices are expedient in the context and provide no precedents for the future, should a similar crisis arise.