Delbanco's attitude toward the original abolitionists is ambivalent. Moreover, he views with some sympathy those who, despite being opposed to slavery, declined to join the abolitionists' ranks. He closes with a quotation from John Jay Chapman, who spoke of "the losing heroism of conservatism" with reference to "New England judge[s] enforcing the Fugitive Slave Law" (e.g., Lemuel Shaw) despite their personal opposition to it (pp.54-55).
My attitude to the abolitionists is more positive than Delbanco's, but I think he makes some interesting points even if I'm not persuaded by them. Toward the end of the essay he provocatively compares the Civil War to recent (and not-so-recent) American wars abroad (Vietnam, Afghanistan, Iraq). I don't think these comparisons work. A quote or two will indicate the tenor of his argument.
He writes (p.43):
...[I]f we imagine ourselves living in the America of the 1850s, how sure can we be of our judgment on the question of intervention in what people of advanced views today might call "the indigenous culture" of the South?
Would we have regarded the firing on Fort Sumter as the abolitionists did -- as a welcome provocation to take up arms against an expansionist power? Or would we have regarded it as a pretext for waging war, akin to that notorious event in every baby boomer's memory, the Gulf of Tonkin incident? If we could have known in advance the scale of the ensuing carnage, would we have sided with those who considered any price worth paying to bring an end to slavery? Or would we have voted for patience, persuasion, diplomacy, perhaps economic sanctions -- the alternatives to war that most liberal-minded people prefer today in the face of manifest evil in faraway lands?
He pushes the point a little further (p.44):
Most of us live quite comfortably today with our knowledge of cruelty and oppression in nation-states whose exports are as essential to our daily lives as slave-grown cotton once was to the "free" North--yet few of us take any action beyond lamenting the dark side of "globalization." Are we sure we would have sided with those who insisted that all Americans--even if they had never seen, much less owned,a slave--had a duty forcibly to terminate the labor system of a region that many regarded, to all intents and purposes, as a foreign country? None of these questions yields an easy answer--but they should at least restrain us from passing easy judgment on those who withheld themselves from the crusade, not out of indifference but because of conscientious doubt.An obvious problem with this line of thought is that although the South might have been seen in the North as a foreign country, the South was in fact part of the same country. As Delbanco himself observes earlier in the essay, Lincoln's original war aim was to preserve the Union, not to abolish slavery. It was only in the summer of 1862 that Lincoln's "mind was opening to new possibilities" (p.13), leading him to free the slaves in the Confederate states but not in border states that had remained in the Union.
Another point is that the Gulf of Tonkin incident was, at least on some accounts, completely manufactured: "North Vietnamese gunboats were probably operating in the area [of the U.S. destroyers Maddox and Turner], but no evidence has ever been produced to demonstrate that they committed hostile acts" (G. Herring, America's Longest War, 2d. ed., p.120). By contrast, there is no doubt that Fort Sumter was fired upon.
Then, too, it is far from clear that going to war to preserve an independent South Vietnam (i.e., independent of absorption into the Communist North) constituted in practice an especially noble goal, given that South Vietnam's rulers, from Diem to Thieu (and pre-Diem as well), were not exactly paragons of democratic legitimacy. By contrast, going to war to preserve the Union seems considerably more justified -- though not, I concede, an open-and-shut case. And to be sure, the Civil War proved very costly in terms of lives and I agree that has to be weighed (cf. Delbanco, p.54).
All this doesn't answer Delbanco's question of how sure we can be of our judgments had we been living in the 1850s. But it does suggest that some of the comparisons he draws are more than a bit strained.
Note: Delbanco's essay, originally a lecture, was published with several responses. I've looked at the responses but not properly read them.