No state shall keep troops or ships or enter into external compacts?
I think you mean Section 8, paragraph a bunch, about calling forth the militia to prevent insurrection? The issue here is that it’s not an insurrection - that’s when a small group is rebelling against the legitimate authority. What actually happened was that that same legitimate authority (the state) no longer recognized a higher authority (FedGov) it ceded some powers to. It simply revoked giving those powers to the US, and instead reclaimed them for the state / ceded them then to the Confederacy.
Article I, Section 10, paragraph 1 “ No state shall, WITHOUT THE CONSENT OF CONGRESS enter into any Treaty, Alliance or CONFEDERATION;....paragraph 3; No State shall...enter into any Agreement or Compact with another state, or with a foreign Power, or engage in War....
Pretty clearly the Founders anticipated this as did Washington who warned of the danger of secession in his Farewell Address.
Andrew Jackson said that he would hang the leaders of South Carolina’s Nullification movement if they tried secession.