The American Civil War was not a true war in the sense that the Union Government held the position that secession from the Union was illegal and military force was used to restore the union by defeating in battle the military forces of the illegally rebelling states. No Southern ambassador or diplomat was accorded any status by the Union so an armistice or peace treaty was never an option because that would legitimize the Confederacy as an actual Nation. The legal right for armed force lay with the Constitution of the United States, which the Union interpreted as unbreakable. The actions of the Southern states were therefore illegal (according to the Union) because they were attempting to drop the Union as their form of Government, which is considered rebellion or insurrection.To have declared war would have required recognizing the Confederacy as a separate sovereign nation which war could be declared against; being viewed instead as an insurrection, war could not be declared.
The festivities began when the South shot first at Fort Sumter. A valid case may be made that the subsequent conflict was just the natural process of the Union ending a fight it did not start.
“which the Union interpreted as unbreakable”
That is the money quote. The intolerant chose intolerance and those not agreeing to the intolerance were crucified. A real great consensual governing document that Constitution. One could always argue that the Constitution is no longer a governing document since 1861. I know Obama and his friends think this way. Hey, they like Lincoln, might be right. All they need is an army of immigrants and more guns than the freemen.