On April 12, 1861 with the firing on Ft. Sumter in Charleston, South Carolina President Lincoln issued an executive order to the states to call out their volunteer militias t0 put down this rebellion. This caused the last holdovers in the south to choose sides and many chose to defend their home states against this "aggression" from the North.
Most famously U.S. Army Col. Robert E. Lee of Virginia, who had been offered command of the Federal forces, assumed command of the Northern Army of Virginia in 1862 and fought for the Confederate States of America. His divided loyalties was just one example of many and a bloody civil war of brother against brother ensued. The American Civil War, or the War between the States, lasted for four years and the fighting cost the lives of over 600,000 Americans on the battlefield.