To answer briefly, the underlying cause of the US Civil War was answering the question: “Is perpetual slavery the proper status of American-born blacks?” In saying “proper status,” I recognize that unfree American-born blacks already had a legal status at the time as slaves, However, the anti-slavery/abolitionist movement in the North increasingly questioned the “Great Compromise” embedded in the Constitution, first on economic grounds then, with increasing vitrol, on moral grounds; ultimately calling into question the Christianity of Southerners who supported slavery.
For really masterful in-depth treatment of the question and convolutions of pre-Civil War US politics, I recommend:
The Impending Crisis, 1848-1861, by David M. Potter
http://www.amazon.com/gp/product/0061319295?psc=1&redirect=true&ref_=oh_aui_detailpage_o03_s00
“David M. Potter (1910-1971) was a professor of history at Yale and Stanford universities. He was posthumously awarded the 1977 Pulitzer Prize for History for The Impending Crisis, which his Stanford colleague Don Fehrenbacher completed and edited.”
This book is well written, very heavily footnoted (almost annoyingly so in places), inexpensive (in paperback) and LONG (650+ pages). It has also the virtue of being concentrated on the years running up to the war and stopping when the war begins. In fact, the author’s entire account of the actual war is limited to the last two pages of the book.
My 2 cents.
A switch of European trade into southern ports would have caused massive economic damage to New England.
The Federal Government made it clear that they were willing to put up with slavery... they just weren't willing to put up with an independent South.