A couple of points:
"The requirements of nationhood" hardly supercede "the Law of the Land." The Constitution of the United States is of greater importance when it comes to the question of State secession than are any international conventions - and the Constitution nowhere prohibits secession.
Also, it is quite obvious that, had the federal government remained within its constitutionally-defined bounds and not opposed secession with military force, the Southern Republic would have had absolutely no difficulty obtaining foreign recognition "as a sovereign nation."
;>)