[arrogantsob]

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.

[Svartalfiar]

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....

And these only apply to states within the Union. Once a state has seceded, any restrictions on the state in the Constitution no longer apply to that state. It's a sovereign entity, completely separate from the US. It has retaken these powers it ceded to the US when it joined the Union.

As for Washington, he warned against several things, such as geographical fractures (N-S and E-W), political fractures (two-party system), and foreign influence being the big three. He said we'd have to work to keep the Union together, never saying it was permanent. His address implies a full belief that states can leave the Union.