If Davis & Co. had their way, they'd take the national capital and everything up to 30 miles from Philadelphia.
They'd be at the Ohio River and 100 miles from Lake Erie -- a convenient point to choke off east-west transportation.
They'd find a way to detach the West Coast and New York City and work mischief in other parts of the country.
What would happen to Kansas, New Mexico, and other western territories would be anybody's guess.
Not to mention Confederate expansion southward into the Caribbean.
Lincoln had to take a stand to prevent the country from falling apart. That didn't have to mean war -- but war was what Davis wanted.
The South did indeed pose a threat to the Union, and the South did indeed invade the North. More than once. Lee took the Army of Northern Virginia into Pennsylvania in a vainglorious roundabout attempt to attack Washington, DC. Had he succeeded, France and England would have stepped in on the side of the South, and the United States of America would have been relegated to the ash bin of history. Instead, Lee f'd up and got his ass handed to him at Gettysburg. Deal with it. Lincoln could whup you hand to hand, he could whup you in court, he could whup you in an election and he could whup you in a Civil War. He could out wrastle, outsmart, out write, and generally best any Southerner who ever lived. He was the greatest American who ever lived.