Well Lincoln also tried to be "neutral" on the slavery issue, but they didn't believe him. (See Corwin Amendment.)
By contrast, Jackson quashed the Secession Crisis of the 1830’s (at least two decades) by taking a strong executive hand.
He threatened to kill John Calhoun, and of course to use the army to quash secession. With Jackson, these were no idle threats, because he was known for his rash impulsiveness and his willingness to engage in violence.
Ironically, they would have had a better chance seceding in the 1830s when the North was weaker.
Buchanan asserted the opposite of Federal executive power in this regard.
He may have believed he had no authority to oppose secession. A lot of people believed that States had a right to secede, and Buchanan was possibly one of them.
He held that the States could do anything they wished about “their” domestic arrangements, and that the Federal Government was essentially powerless.
Lincoln also held this position and reiterated this point many times. He often said he had no power to interfere with slavery where it existed.
Buchanan had been out of the country as a diplomat during much of the key wrangling over slavery in Congress, and perhaps did not grasp the intensity and danger posed to the Union.
160 years in hindsight, and *I* do not grasp the danger posed to the Union. How would Southern secession damage the Union? And damage it worse than killing 750,000 people, creating an all powerful Federal government, evaporating 5 billion in capital, and wrecking the South's economy for the next 100 years?
Letting the states leave in peace would have done more damage than that?
“Let the South secede” vs “horrors of civil war” ignores other possibilities. Such as: With a mix of Jackson’s strength and Clay’s finesse/compromise, the slavery can might have been kicked down the road a couple of decades, after which mechanization would have made slavery unprofitable in the northern tier of what became the Confederate States. Without those States, Southern secession would not have been viable. Slavery would have petered out without a war.
An independent South, with it’s government modelled on the failed Articles of Confederation; very undeveloped economy; a severe shortage of money; worn out land; new production in India and Egypt undermining the cotton trade; millions of enslaved people to feed and deal with; etc. would have become dependent on Great Britain to the point of functioning as its colony. That would be a major problem for the US. The logic of the South’s third-rate leaders required expansion, which would have definitely been a problem for the US, with the two “countries” on path to war. The enslaved population would have continued to grow, with diminishing profitable work, until the slavers could not contain them. Immigrants would choose the free North and avoid the South, where the yeoman farmer model would die out. Add in farm mechanization, poor standards of education, and the growing gap between industrialized and agrarian nations and the South would have ended up like Haiti. That would have been quite a problem for the Union.