“...slavery could not possibly have been the primary issue inspiring secession due to the fact that four slave states remained in the Union.”Here we have I think a decent argument, except that it ignores the possibility that slavery could have been the primary (i.e. the largest of many) issue inspiring secession and yet was not powerful enough to bring about secession everywhere it existed. There is no need to presume that slavery was as important in Missouri - which officially (if arguably***) did not secede - as it was in Alabama, which did, just because they were both slave states.
This question is probably better examined on its head: is there a meaningful distinction between slave states who seceded and those who did not? Is there a trend we can discover that might explain that distinction? The answer to both is yes: it is the slavery "density" of the resident population. In Missouri the free population was 9 times that of the slave population (1,067,081 vs. 114,931), while in Alabama (519,121 vs. 435,080) they were nearly equal. In South Carolina, the hotbed of secession, there were actually more slaves than free (301,302 vs. 402,406).
This is not cherry-picking the data for a couple states that fit. The total ratio of free to slave in the border states was 6.4:1 free, whereas in the Upper South it was less than half of that: 2.5:1 free. In the Lower South, the population was even less free, at a ratio of 1.1:1 free. Therefore one could argue - I do - that slavery was a significantly less important issue in those slave states that did not secede****, or put another way, that it proved decisive in those that did. It is no coincidence that those states with a lower free-to-slave ratio tended to secede first or that every state that seceded had a lower free-to-slave ratio than Kentucky, the lowest-ratio slave state that did not, at 4.1:1.
Missouri for example had a large, free, immigrant population that Deep South states did not, and that population was pro-Union and certainly less tied to slavery than the older "planter class." It is of interest that those areas closest to Kansas, where slavery was most prevalent, were more secession-minded than was, say, Saint Louis, which received a huge German influx after 1840 and doubled in size to 160,000 between 1850 and 1860. The Dutch, as they were called, were generally considered anti-slavery (though their views varied) and made up a significant percentage part of Missouri's 100,000 troop allocation to Union armies. Missouri provided (if I remember correctly) 35,000 or so troops to the South. While Missouri was a "slave state" it was not so to the same extent as Alabama, and even within it, secessionist sentiment was generally proportional to the prevalence of slavery.
Now Vox does come up with an interesting analogy, which can only be done justice presented in full:
Suppose you want to fly a flag, as permitted by the rules of your homeowner's association, but the committee that runs the homeowner's association suddenly decides they don't want you to fly one. They show up at your house unannounced to inform you of their decision, then barge into your bedroom in order to confiscate any flags that they might find. If then you punch the head of the committee in the face, was your desire to fly a flag the cause?That’s a bit unfair historically as it presumes the South punched the North only after the north invaded their bedroom and ignores the issue of secession altogether. It would be better analogized this way:
Suppose you want to fly a flag, as permitted by the rules of your homeowner's association, but you are certain that the incoming president will move to eliminate flags, even in your home. You then attempt to leave, only to find yourself physically restrained in your home by the association police.Since you had in the past used homeowners’ association rules (e.g. the Fugitive Slave Act) against your neighbors, it is obvious that you have no problem with the rules in principle, your current protestations aside. But if you’re trying to leave now that a power you have used in the past might be applied against you, what can one conclude but that your desire to continue flying your flag is the decisive reason for your attempt to leave?
* the reasons for the war itself are a completely different story, and I am also skipping the question of state sovereignty. Sovereign or not, states still have the right to secede. The Declaration of Independence tells me so.
** historians are as faddish as 13-year-old girls. This is further complicated by the fact that slavery was hidden by euphemism and wrapped up in other issues by public speakers for decades. If you have ever listened to a speech praising 'choice,' you get the picture.
*** Like other border states, there was enough fraud and force to feed the IRS for a year. But it only would have worked under circumstances that will become clear.
**** Thereby allowing the aforementioned fraud and force to succeed.