... when they seceded over the issue of slavery.
Now suppose that Lincoln had acquiesced, and the South Carolina militia had not decided to start a war at Fr. Sumpter.
You really think that war would not have started in the West, as the new Confederacy came into conflict with the United States over expansion of slavery into the territories that would now be claimed by both?
The FACT is that secession over the issue of slavery had been brewing for decades. The FACT is that shots had already been fired in Kansas and elsewhere over whether slavery would expand into new territories.
Secession would not have prevented war. All that Ft. Sumpter did, was to start the war on the Eastern Seaboard, instead of out west.