Where was the treaty signed?
There was no treaty signed to end the Civil War. The surrender at Appomattox Court House was a military surrender of an army which was surrounded. The Confederate government never surrendered and even had it wanted to the United States government would likely not have accepted. To do so would have legally acknowledged the existence of the Confederate States of America and would have legitimized it and given it certain legal status internationally. Treaties are between two nations and the U.S. would never concede the legal existence of the Confederacy – even though it had a government, armies, taxes and all the trappings of a modern government.
