When the question of whether the Union could have avoided the Civil War by purchasing the slaves' freedom, most people (rightly) point out that this would have been completely unfeasible because (1) purchasing every slaves' freedom would have been astronomically expensive; and (2) the Confederacy would have never voluntarily agreed to end slavery under any circumstances.
I believe those reasons (particularly #2) are correct, and I also doubt that the eminent domain power would have been sufficient for the Union to compel any owner who wanted to keep his slaves to submit.
But in addition to the aforementioned reasons, once the South declared their intent to secede, was the Civil War then unavoidable simply because the Federal government could not allow the threat of secession to be an acceptable method of negotiating differences between the states and the Federal government? Didn't Lincoln need to go to war when the South acted on the threat of secession, lest the Federal government be forever at the mercy of any state or group of states that threatened to secede?
Here's a relevant quote from Lincoln's First Inaugural Address:
It is seventy-two years since the first inauguration of a President under our National Constitution. During that period fifteen different and greatly distinguished citizens have in succession administered the executive branch of the Government. They have conducted it through many perils, and generally with great success. Yet, with all this scope of precedent, I now enter upon the same task for the brief constitutional term of four years under great and peculiar difficulty. A disruption of the Federal Union, heretofore only menaced, is now formidably attempted.
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.
Again: If the United States be not a government proper, but an association of States in the nature of contract merely, can it, as a contract, be peaceably unmade by less than all the parties who made it? One party to a contract may violate it—break it, so to speak—but does it not require all to lawfully rescind it?
Descending from these general principles, we find the proposition that in legal contemplation the Union is perpetual confirmed by the history of the Union itself. The Union is much older than the Constitution. It was formed, in fact, by the Articles of Association in 1774. It was matured and continued by the Declaration of Independence in 1776. It was further matured, and the faith of all the then thirteen States expressly plighted and engaged that it should be perpetual, by the Articles of Confederation in 1778. And finally, in 1787, one of the declared objects for ordaining and establishing the Constitution was "to form a more perfect Union."
But if destruction of the Union by one or by a part only of the States be lawfully possible, the Union is less perfect than before the Constitution, having lost the vital element of perpetuity.
It follows from these views that no State upon its own mere motion can lawfully get out of the Union; that resolves and ordinances to that effect are legally void, and that acts of violence within any State or States against the authority of the United States are insurrectionary or revolutionary, according to circumstances.
I therefore consider that in view of the Constitution and the laws the Union is unbroken, and to the extent of my ability, I shall take care, as the Constitution itself expressly enjoins upon me, that the laws of the Union be faithfully executed in all the States. Doing this I deem to be only a simple duty on my part, and I shall perform it so far as practicable unless my rightful masters, the American people, shall withhold the requisite means or in some authoritative manner direct the contrary. I trust this will not be regarded as a menace, but only as the declared purpose of the Union that it will constitutionally defend and maintain itself.
What this boils down to is that, in Lincoln's view, it was his responsibility as President to defend the integrity of the US and its Constitution by preventing disunion.
While the relationship between state and federal government is obviously relevant here, I do not interpret these statements as expressing a belief on Lincoln's part that the federal government must demonstrate its superiority over state governments. Rather his position as President (a federal position, yes) obligates him to maintain the integrity of the governing system as a whole, that system being composed of divided powers and responsibilities between various federal and state institutions. Lincoln's actions in preventing secession were not necessarily equivalent to the forced subordination of state governments because the specific and relative office within the US system of government that he occupied obligated him to resist these kind of actions.
At least, that seems to be the argument being made here. Whether Lincoln was in fact doing these things, or more broadly whether the federal government could in fact defend against secession without subordinating state governments, is certainly debatable. There's no question that the Civil War resulted in a large shift in the relative power of the federal government over state governments.
I have been trying to figure out for some time exactly how centralized the United States government was prior to the Civil War, but the fact remains that it became more centralized as a result of the Civil War. However, I reject the knee-jerk reaction that some people have to this fact, specifically their assumption that a federal power grab must be the only explanation for why this centralization took place.
In contrast I tend to argue, as several other scholars have done, that the Civil War was in part a constitutional crisis in the US. Disunion was not first attempted, threatened, or discussed in this country in 1860, nor was the relative power between state and federal governments first debated or fought over. Part of what made these debates so contentious and frequent was that there was no easy way to answer them. Due to the varied arguments and diversity of viewpoints expressed during the ratification process of the Constitution (as in, different people supported and interpreted the Constitution in different ways when they voted to ratify it, not to mention that some states, like Rhode Island or Georgia, were barely involved in the Constitutional Convention), exactly what the Constitution meant was uncertain. Supreme Court decisions helped clarify these issues but this process was largely subject to specific cases which were brought before it; The Supreme Court can't just decide to rule on the concept of secession one day because it feels like it. Some of these constitutional questions, like secession, were only resolved once the Civil War forced them to be resolved, and that means that some of the resolutions were determined not be courts but by who won the damn war.