And I think you have heard that one of the framers, a certain Thomas Jefferson, forecast and anticipated slavery, as the "rock upon which the old Union would split." (To paraphrase Alexander Stephens in his Cornerstone Speech.)
A century and a half later, it may yet be.
By the way, I disagree with U.S. Grant’s statement. I believe the Founders did forsee the possibility of an attempt at an armed dissolution of the Union, and were willing to submit the nation to a war to preserve it.
Reread the Constitution as adopted. Count the references to “rebellion” and to how to handle cases of Treason. The document is laced with them, a reference in virtually every article. Also, consider Article VI, Sec. 3, Clause 1:
“The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution....”
Not just Federal office holders, but every single state and local officer down to 3rd shift assistant mens’ room attendant, is required to take an oath upholding the Constitution.
They clearly intended the Union to be indivisible, contemplated that armed force may be necessary to preserve it, and were willing to apply it.