Months after Britain seceded from the E.U., the government of the former state went rogue in intending to pass a law that would unilaterally change, and thus violate, the terms of the post-secession trading agreement between the UK and its former union. The bill proposed no new checks on goods going from the Northern Ireland region to the other regions of the UK. Whereas the British prime minister was claiming that the full sovereignty of the former state meant that Parliament could unilaterally change the terms of a treaty, the European Commission was saying, in effect, that an agreement is an agreement. I contend that the Commission was correct. Moreover, even before the UK seceded from the European Union, an obsession on sovereignty (then, states' rights) rendered Britain vulnerable to failing to grasp the costs.
I submit that a treaty is no longer valid if one party uses its governmental sovereignty to unilaterally alter the terms. The E.U. could have replied that if the former state passes its bill, then the trading treaty would be null and void. The UK wanted a trade treaty, so that government would have to start from nothing in negotiating a new one should that government violate the one already agreed to prior to the secession. Fortunately, the secession itself was not contingent on the post-secession trading treaty going into effect, for Britain's insistence on its full sovereignty even as the state was semi-sovereign as a state in the E.U. was problematic.
While the UK government's insistence on its full governmental sovereignty was correct at the time the proposed bill was proposed in 2020 (i.e., after secession), the claim of full sovereign of any E.U. state, including the UK before it seceded, would be false. The rational flaw that resides in the contention that the trade treaty would still hold even if one of the parties unilaterally changes the terms is also in the contention that a state of a union can unilaterally change the terms of federal law, as South Carolina had done in the U.S. in the early 1830's.
Put another way, whereas the British government was correct that a trading agreement going into effect after secession counts as a treaty, which a government's legislature can change because it is sovereign, to make the same claim regarding federal law in a political union would not hold. So when David Cameron claimed when Britain was an E.U. state that the E.U. was just another network of (sovereign) countries to which Britain happened to belong, he was wrong in implying that the British Parliament held full sovereignty. He was conveniently ignoring the fact that the states had delegated some sovereignty to the E.U., and that laws passed on the federal level were not treaties relative to the sovereignty retained by the state governments.
Even though Britain was no longer a state of the E.U. in 2020 when the Parliament was considering a bill to unilaterally change what was in fact a treaty, it seems that the need to continue to emphasize sovereignty was a bit overdrawn, and not in the former state's economic interest. For just as one party to a treaty can change it unilaterally, the other party can walk away, tearing up the treaty and legitimately claim that no agreement exists.
