The Supreme Court, of the corporate United states, decides Geofroy v. Riggs:

       The treaty power, as expressed in the Constitution, is in terms unlimited except by those restraints which are found in that instrument against the action of the government or of its departments, and those arising from the nature of the government itself and of that of the States.  It would not be contended that it extends so far as to authorize what the Constitution forbids, or a change in the character of the government, or in that of one of the States, or a cession of any portion of the territory of the latter, without its consent. 

       [added 5/15/2022] thanks to Bill Holmes for this entry.