The statement was made that there were NO slaves in New Mexico Territory and Utah Territory in 1860. That is wrong. In fact there was a low level of aftican slavery in both territories, and a greater level of indentured servitude (peonage) in the former Spanish/Mexican areas.
And if you had read Rhett's broadside about the subject, you would see that he envisioned large slave populations in both areas due to mining - because slaves were just perfectly suited to mining! The reason for refuting the post was that, after the Dred Scott decision, anywhere that slavery had a foothold in a territory, it automatically became a point of contention. Those territories, and including the Kansas and Nebraska Territories, represented a huge area open to potential slavery. The southern leadership was not to be denied their slave territories and expansionist goals.
correction: nc's post was #238
Untrue, having "a slave" anywhere was not dispositive of the question of slavery's acceptance -- it certainly wasn't true in Kansas or California. The Southern States walked away from the territories north of the Missouri Compact line, and Kansas and California, when they seceded. They might realistically have entertained some thoughts about New Mexico and the Indian Territory becoming part of the Confederacy eventually, but Colorado became a State during the war, and California and Oregon were already States -- Oregon having been populated by Northerners mostly (as its voting patterns still show today -- one of the things realized by Richard Nixon when he reviewed young Kevin Phillips's map of national electoral patterns, which he compiled as a teenager).
Slavery, as the document I quoted shows, was excluded from California by freesoil interest through devices very like those complained about by freesoilers in Kansas, when the Lecompton convention sat. Of course, we don't hear about Southerners' having received a raw deal in California, which was, as Rhett says, very well suited to plantation agriculture -- or any other kind of agriculture, since it has become one of the breadbaskets of North America.
Nevertheless, the Southerners were successfully shut out of California by a cabal, which obtained California's admission to the Union as a freesoil State. And so California was not on the table when the Southern States left the Union.
The Southerners were, I'm sure, perfectly aware that they couldn't just claim large territories and walk off with them, without some sort of resolution of the Territories' status between the Confederacy and the United States. But the Confederate commissioners attempting to discuss issues created by secession were repeatedly rebuffed by Lincoln, and so I'm sure it became pretty obvious that, the United States being on a war footing, there would be no negotiation where the Territories were concerned.
The secession convention delegates had to be aware of this possibility, that secession might mean the abandonment, in practice, of all access to the Territories. But they went ahead and voted for secession anyway -- truncating that leg of your "it was all about slavery" argument.
Excellent post.