"To do less would have been to violate his oath of office in my opinion"

What part of his oath would that be?

I presume that maybe you are referring to preserve, protect, and defend the Constitution of the United States, part, but I am unsure how that applies to surpressing the independence of sovernign states, who were not attempting to overthrow the Government of the United States, and therefore were NOT in rebellion. Independence is not rebellion, but a God given right of all men, according to Thomas Jefferson.