The powers that the federal government has come from the people, or at least that is where it is supposed to come from. The US Constitution was created by the people for the people. The people of the Confederate States chose to leave the Union. Now as we all know, right has been taken from the states at the point of a bayonet.
Nothing in the in the constitution prohibits secession, there were no laws against it at least in 1860. I went to this site http://en.wikipedia.org/wiki/Tenth_Amendment_to_the_United_States_Constitution to get a better understanding of just what you are talking about. I did notice that there is something called "Forced participation or commandeering" and acts such as this are declared unconstitutional. No doubt the Confederate states were forced back into the Union, and then this act should also be declared unconstitutional.
In regards to treaties, I don’t think the Federal government ever had any trouble breaking an any treaties. Just ask the Red Man about some instances I am sure he can provide us with a few instances. So why be right when you can use might?
GP