"what happens when the sovereign side unilaterally declares, "Sod this!" How would the other side be able to make them adhere to the law?"
We already know the answer to that. You can't, as they just say some magic words like 'abracadabra' or 'Supercalifragilisticexpialidocious' or even 'Paedophile' or 'National Security' or some such bollocks.