That is an extremely Machiavellian point of view. We fought WWII to defeat an evil that would seek to destroy all peace loving people. The invasion of the South was to force southerners to remain under federal control. The positive byproduct of an end to slavery was already on its way to being quickly acheived, and would have been accomplished in a way that did not destroy a society and leave the freed slaves in a position of serfdom. Had Lincoln been so concerned with obeying the Constitution he would have seen that in Article 1, Section 10, the rights denied to the states, it does not prohibit secession.
I suppose we can all agree that whatever happened there wouldn't be slavery on any part of the continent in the 21st century, but people in 1860 didn't have the same point of view. For a large number of them, the victory of freedom or slavery was at stake in their lifetime and they sides chose accordingly.
Another thing that we ought to be able to agree on is that any emancipation undertaken by the CSA would precisely have "left the freed slaves in a position of serfdom." And it would probably have been the gradual compensated emancipation (possibly with subsidies for emigration) that was discussed earlier in America and other new world countries. It could well have left people in bondage of one sort or another (slavery or serfdom) until well into the 20th century.
Or do you really think the victorious Confederacy would have abolished slavery and immediately admitted the freedmen to all the rights of citizenship and equality with the Whites? That would indeed be quite a fantasy, and would go against so much of what was said at the time!
Was the war worthwhile? I don't know. It's hard to say that those losses could ever be justified. I simply say first, that people going into the war didn't know what we know now, and second, that not all the blame for the war can be laid on the North. We can turn the question that's usually asked around: if secessionists had known the costs that war would bring and the eventual fate of the country would they have been so quick to break with the union?
Article 1, Section 10 forbids states from making treaties or alliances or separate confederations with each other. Put that together with the Supremacy clause (Article 6, Clause 2) and you have a pretty good argument against the Constitutionality of secession. At the very least, Southern leaders shouldn't have been so rash and hotheaded in acting unilaterally, but should have worked through Congress if they really wanted to leave the Union.