If the EU will not modify the proposed Brexit agreement to allow the UK to end a backstop when it chooses to do so, it seems unlikely that it could command a Parliamentary majority to implement it. In which case the UK would leave on March 29th under WTO rules.
David Singh Grewal, a professor of law at Yale University, and Richard Tuck a professor of government at Harvard University, have written an intriguing piece in the Irish Times suggesting a possible course of action the UK might then undertake, much to its advantage, and providing a solution to the apparent impasse.
They point out that WTO rules allow for a “national security exception.” It is in article 21 of the general agreement on tariffs and trade (Gatt). “Nothing in this agreement shall be construed . . . to prevent any contracting party from taking any action which it considers necessary for the protection of its essential security interests . . . taken in time of war or other emergency in international relations.”
These are somewhat uncharted waters, and it isn’t as clear as one might like what would constitute an “essential security interest” or an “emergency in international relations,” but the UK could argue that free access of goods across the Irish border was indeed vital for our security interest, which is true, or would prevent an emergency in international relations, which is also true.
It would not be completely unprecedented, however, because President Trump has claimed that the preferential tariffs imposed on some foreign goods are need to protect the security interest of the US.
If this reasoning were sustained it means that the UK could, under WTO rules, allow goods from the Irish Republic free access across the border into Northern Ireland, without allowing goods from the rest of the EU similar free access until a free trade deal had been completed. There would thus be an open border that honoured the Good Friday Agreement.
How the Republic of Ireland and the EU chose to respond to such a unilateral initiative remains to be seen, but it would be to the advantage of both to accept it. They would not be able to prevent the UK from doing this, even should they perversely wish to do so. The UK could use this approach to prevent a backstop from inhibiting its future freedom, or from requiring the consent of the EU to end it.
This would seem to be a valid and attractive alternative if no deal can be agreed that can pass through Parliament.