It actually started in Congress with the debate over the Southern (Slavery) States arguing over the Non-Slavery States returning run away slaves. The Non-Slavery States made it clear they were not going to return the run away slaves. This ensued a battle in Congress and the Non-Slavery States stated the Slavery States could not force the Non-Slavery States to give up the run-away slaves.

This became a battle between States Laws. In order to stop the argument in Congress, it became a Federal Law where slavery became illegal in the United States of America. As soon as the Federal Law was passed this angered the Slavery States and the Southern Slavery States wanted to become separate and begin their own country. If the Slavery States would have won the United States would have been split into two different countries.

The Confederate States elected their own President and the Union stood by the elected President of the United States, who at that time was President Abraham Lincoln.

The American Civil War (1861–1865) was a major war between the United States (the “Union”) and eleven Southern slave states which declared that they had a right to secession and formed the Confederate States of America, led by President Jefferson Davis.

The Union, led by President Abraham Lincoln and the Republican Party, included all of the free states and four slaveholding border states that were later joined by West Virginia. Republicans opposed the expansion of slavery[1][2][3] into territories owned by the United States, which increased Southern desires for secession.[4] However, Republicans (and the previous Democratic administration under Buchanan) rejected any right of secession.

Fighting commenced on April 12, 1861, when Confederate forces attacked a United States (federal) military installation at Fort Sumter in South Carolina, the first state to secede.

This one of the reasons why Federal Law precedes Statutory Law (State Law).