Unsustainable? Why? More States joined the Confederacy when Lincoln called upon them for troops to invade and conquer the Southern Confederacy. They were unwilling to join Lincoln in pissing upon the Declaration of Indepedence's concept of political sovereignty.
Funny how the Chi-coms echo Lincoln on the subject of secession and the rights of people to determine their own government and poltical bonds.
For most of early 1860 the Upper South and Border states were on the fence with regard to secession. Had they remainined in the Union it is fairly obvious that the rump Confederacy would not have been large enough to be viable in the long run, as the chance of future conflict between it and the US approaches 100%.
Think of the Confederate leaders as similar to Serbian and Croat extremist leaders during the breakup of Yugoalavia. Most people, in all the ethnic groups, just wanted to get along. The extremists precipitated war, as in the crunch most would support "their people." War has the effect of forcing people to take sides. The Confederacy, IMHO, initiated war to force Upper South and Border states to pick a side.
They were unwilling to join Lincoln in pissing upon the Declaration of Indepedence's concept of political sovereignty.
This is an example of the logical fallacy of begging the question. All Americans of the time believed in the principles of the D of I. Some claimed it gave states the right to secede. Others strongly disagreed. Asserting a claim does not constitute proof or even evidence.
Whenever two groups strongly hold mutually exclusive positions, the conflict can be resolved in only two ways: mutual agreement to accept the ruling of some authority (in our system usually that of a court), or by violence.
The seceding states chose to not take the first approach. Eventually, they decided to try the second, and they came very close to success.