Emancipation

150 years ago today, President Abraham Lincoln signed a "preliminary" Empancipation Proclamation, letting it be known that all persons held as slaves within the states still in rebellion on January 1, 1863, would become free. (Apparently, Lincoln had dedided on this course as early as July but felt he had to wait until a major American victory over the rebels. That happened at the Battle of Antietam on September 17, 1862, widely considered to be the bloodiest single-day battle in American history, with some 23,000 on both sides.) 
Among other things, this further highlighted the moral superiority of the Union cause and likely contributed further to keeping Britain and France from aiding the Confederacy. Even before the Emancipation Proclamation, it would have been hard to advance a morally convincing case for secession. In most coherent ethical frameworks, the burden of moral proof to justify a rebellion rests with the rebel, and the bar for such justification is necessarily and properly high. Theories of state sovereignty should have been buried along with the Articles of Confederation the day the U.S. Constitution took effect on in 1789. As Lincoln himself said, in his 1st Inaugural Address:

"I hold that in contemplation of universal law and of the Constitution the Union of these States is perpetual. Perpetuity is implied, if not expressed, in the fundamental law of all national governments. It is safe to assert that no government proper ever had a provision in its organic law for its own termination. Continue to execute all the express provisions of our National Constitution, and the Union will endure forever, it being impossible to destroy it except by some action not provided for in the instrument itself."

That such a fantastic notion as state sovereignty still survived with a lingering patina of respectability fo so long and required such a catastrophic conflict to be suppressed says something, I suppose, about the political and moral limits of the original constitutional compromise.
In the end, of course, theories of state sovereignty served only as a sort of political theory fig-leaf for what the conflict was ultimately all about. At his 2nd Inaugural, as the terrible war was clearly coming to a close, Lincoln reflected on its origins:

"Both parties deprecated war, but one of them would make war rather than let the nation survive, and the other would accept war rather than let it perish, and the war came.

"One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was somehow the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union even by war ..."