Abraham Lincoln had insisted from the beginning that since war is a condition that exists only between two sovereign nations, and since the Confederacy was in his view only an insurrection, no actual war legally existed between the North and the South. (The United States, for instance, never declared war against the Confederacy.) So it was not quite clear that Lee could be, by Lincoln’s definition, guilty of levying war.

https://www.washingtonexaminer.com/weekly-standard/the-trial-that-didnt-happen