Grant was simply under-educated, and therefore ignorant and unfamiliar with the peoples, politics, and events of the times. Here is a quote with the understanding of a witness, not the rhetoric of a general turned politician:
From the Charleston Mercury, December 21, 1860:
“The State of South Carolina has recorded herself before the universe. In reverence before God, fearless of man, unawed by power, unterrified by clamor, she had cut the Gordian knot of colonial dependence upon the North—cast her fortune upon her right, and her own right arm, and stands ready to uphold alike her independence and her dignity before the world.
“Prescribing to none, she will be dictated to by none; willing for peace, she is ready for war. Deprecating blood, she is willing to shed it.
"Valuing her liberties, she will maintain them.
“Neither swerved by frowns of foes, nor swayed by timorous solicitations of friends, she will pursue her direct path, and establish for herself and for her posterity, her rights, her liberties and her institutions.
Though friends may fail her in her need, though the cannon of her enemies may belch destruction among her people, South Carolina, unawed, unconquerable will still hold aloft her flag, “ANIMIS OPIBUSQUE PARASSTI’ “ (the South Carolina state motto, “Ready in soul and resource”).
That "worst...cause" was Liberty, the same cause that led the South to victory over the British 80 years earlier.
Grant was a West Point graduate, just as Lee was.
Not entirely.
The editors at the Charleston Mercury agreed. They had anticipated the threat that a Republican victory would pose when in early November they warned South Carolinians and the entire South that “[t]he issue before the country is the extinction of slavery.” “No man of common sense, who has observed the progress of events, and is not prepared to surrender the institution,” they charged,“can doubt that the time for action has come—now or never.” The newspaper editors, like most Southerners saw Lincoln’s election as lifting abolitionists to power, and like most southerners they understood, as they plainly stated, that “[t]he existence of slavery is at stake.”
“What Shall the South Carolina Legislature Do?,”The Charleston Mercury, November 3, 1860. Source