That provision has been violated a couple of times in history. The only purpose by which the United States army can be used "Legally" to maintaine peace is under Marshall Law (I.E. a request from a Governor or other legal authority for assitance to local law enforcement) or Rebellion. The only way that the people of the Southern States were in "Rebellion" is if the Constitution contained a clause which mandated a Perpetual Union, whereby Lincoln would have then been authorized to "Preserve the Union".
I find no such "Prepetual Union" clause in the Constitution and neither did the legal minds of that day, as explained in William Rawle's and other textbook on the Constitution and Constitutional Law. By the way we still don't have a perpetual Union Law even today in the Constitution or Federal Law, that I know of. Can anybody cite such a law?
Yes, I know about the SCOTUS Ruling that says that Secession is illegal, BUT, as far as I know of that ruling was never backed up by any federal legislation following that ruling. That I believe would take an amendment to the Constitution, which I do not believe would recieve the endorsement of 37 states today.