Article IV, Section 3
No, I believe that states that it cannont happen without consent of Congress and the legislatures concerned.
Looking at the records, I can't find a case where the Congress DENIED the secessions.
And I believe Article I applies to states WITHIN the Union.
None of the Southern States that joined the CSA were part of the USA at the time. They did not impose one State as being "subject" to another.
Agreed, and that is the position that I have always taken. Secession is not prevented by the Constitution, unilateral secession is. And the southern states taken their case to Congress, there is no reason why Congress could not have voted to approve their acts of secession. But at least then all the interests of all the affected parties would have been addressed. Issues that may have caused conflict could have been negotiated prior to the separation. And the actual division could be peaceful.
None of the Southern States that joined the CSA were part of the USA at the time. They did not impose one State as being "subject" to another.
Once common feature of the restrictions in Article I, Section 10 is that they are all actions which, if taken unilaterally, could have a negative impact on the interests and wellbeing of other states. That's why Congressional approval is required. States leaving unilaterally, appropriating government property, repudiating the debt built up by the nation while they were a part, cutting off access from the sea via the Mississippi, all had a negative impact on the other states. So on those grounds as well their secession should have been approved by the other states.