One question that rarely gets asked, much less answered, is, Why did Northern leaders refuse to allow the South to leave in peace? Clearly, their refusal to accept the Confederate peace offer had nothing to do with slavery, since secession settled the thorny issue of slavery in the territories in the North's favor, since the North would no longer have to worry about enforcing the Fugitive Slave Law, and since, as even John Nicolay noted, separation would have the effect of moving the Canadian border to the South's northern border. So if slavery was the real issue, Northern leaders should have been thrilled with secession.
If we read Northern newspapers from the time South Carolina seceded until the announcement of the low Confederate tariff, we see a number of editorials in favor of allowing the South to leave in peace. But many newspapers changed their tune very quickly after the Confederacy announced its low tariff. In a matter of days, some Northern newspapers began to warn of dire consequences for Northern businesses if Northern ports had to compete with Southern ports.
We see another clear hint in Lincoln's first inaugural address. He mentioned two reasons that he would invade: to enforce federal authority in the seceded states and to collect the tariff (to collect "duties and imposts"). Interesting. Not one word about invading to free any slaves. In fact, in that same speech, Lincoln mentioned his support for the Corwin Amendment, which would have forever prohibited the federal government from abolishing slavery.
Even if we assume for the sake of argument that the South did in fact secede only to protect slavery, that still leaves us with the fact that the North's refusal to allow the South to leave in peace had nothing to do with any concerns about slavery.
However, the record is clear that the South did not secede merely to protect slavery. Not only did four of the seven Deep South states mention economic complaints, especially the tariff, in their secession documents and/or addresses and speeches, but the four Upper South states initially
voted against secession when slavery and the tariff were the main issues, and they only changed their minds later, after the fall of Fort Sumter, when it became obvious that Lincoln was going to launch an invasion.
For more information about the tariff as a major factor in sectional strife and secession, see
The Tariff and Secession.
