They weren't fighting to defend the institution of slavery directly...

Actually, yes they were, Amos. I'm somewhat surprised you subscribe to this common misconception. Have you ever read any of the contemporary documentation? for instance the South Carolina Ordinance of Secession? Excerpt follows:

" Those States have assumed the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property [in slaves] established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery; they have permitted open establishment among them of societies, whose avowed object is to disturb the peace and to eloign the property of the citizens of other States. They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection.

For twenty-five years this agitation has been steadily increasing, until it has now secured to its aid the power of the common Government. Observing the *forms* [emphasis in the original] of the Constitution, a sectional party has found within that Article establishing the Executive Department, the means of subverting the Constitution itself. A geographical line has been drawn across the Union, and all the States north of that line have united in the election of a man to the high office of President of the United States, whose opinions and purposes are hostile to slavery. He is to be entrusted with the administration of the common Government, because he has declared that that "Government cannot endure permanently half slave, half free," and that the public mind must rest in the belief that slavery is in the course of ultimate extinction."

This is only one of literally hundreds (if not thousands?) of contemporary examples, including the writings of Jefferson Davis, Alexander Stephens, Etc.Etc. They made no bones about the fact they they were fighting to preserve and protect the institution of slavery- moreover they were proud of it.

I often wonder if they would be amused or offended and dismayed by the persistent 20th century efforts to whitewash the fact. Talk about "political correctness"!

Best, Greg