But "the South" did not secede peacefully!
It was unlawful and violent from Day One, December 1860 -- seizing dozens of major Federal properties, threatening Union officials, firing on US ships.
There was nothing "peaceful" about it.
Then in April, 1861, the Confederacy started Civil War by launching a long-planned military assault on Union troops in Union Fort Sumter, and on May 6, 1861 formally declared war on the United States, at the same time sending military aid to pro-Confederate forces in the Union state of Missouri.
Here is a definition of "civil war" which certainly includes the US from 1861 - 1865:
Neither the United States nor any major power on Earth ever recognized the Confederacy as a legitimate independent country.
And China does not recognize Taiwan.
And North Korea does not recognize South Korea.
Georgia does not recognize Abkhazia.
Nigeria did not recognize Biafra.
And Iran does not recognize Israel.
And don't forget, the United States did not recognize Manchukuo.
What does all this mean? Morally, nothing. Economically it means much, especially if a larger power conducts total war against civilians.