What major event happened in the US December of 1860?

The major event that happened in the U.S. in December of 1860 was South Carolina's secession from the Union. On December 20, 1860, a special convention in South Carolina adopted an ordinance of secession, declaring that the state was no longer part of the United States. This marked the beginning of the American Civil War, as other Southern states soon followed South Carolina's lead and seceded from the Union.