"Best endeavours" clauses and the concept of "good faith" are used when parties either cannot, or will not, accept an absolute obligation to deliver a particular result. Even if one party is found not to have tried hard enough, or to have fallen short of the requirement of "good faith", that does not automatically mean that the other can simply walk away. Article 178 of the Withdrawal Agreement (WA) merely provides that if either the EU or the UK is found to have acted in breach of those obligations, then there would be a temporary suspension of parts of the agreement while further negotiations take place. There is no provision for unilateral termination of the WA, or of the "backstop" provided by the Northern Ireland protocol, should that come into effect. From a legal perspective, that position remains essentially unchanged by the additional documents that emerged from the Strasbourg talks on 11 March.
That conclusion was reached by Keir Starmer MP and also (separately) by Lord Anderson, Jason Coppell QC and Sean Aughey in a legal opinion published by the People's Vote campaign. Crucially, it also reflects paragraph 19 of the letter published on 12 March by the Attorney General, referring to the "backstop" arrangement in the Northern Ireland protocol:
the legal risk remains unchanged that… the United Kingdom would have, at least while the fundamental circumstances remained the same, no internationally lawful means of exiting the Protocol's arrangements, save by agreement
With the 12 March "meaningful vote" in mind, a great deal of media attention initially focused on the question of whether the "Joint Interpretative Instrument" (JII) and additional language in the Political Statement agreed in Strasbourg would be "legally binding". However, that is not quite the right question to ask. As instruments agreed between the EU Commission and the UK government there is no real doubt that they are legally binding – or at least that they would become legally binding if duly ratified.
The real question relates to their actual legal effect. Neither document provides a unilateral right for the UK to terminate the WA or to walk away from the backstop. Commitments to explore and to work towards "alternative arrangements" would have legal force, but they oblige the parties only to try – they do not guarantee success.
The Attorney General's letter includes the observation (para 17) that the JII and the UK's unilateral declaration "reduce the risk" that the UK could be indefinitely and involuntarily detained within the Protocol's provisions, but with the crucial caveat: "at least in so far as that situation had been brought about by the bad faith or want of best endeavours of the EU". Legally, the choice of words is significant. They point towards an extremely strong test in relation to the EU's conduct. In essence, they suggest that there would be a breach of the EU's "good faith" obligation only if it were possible to show "bad faith". The burden would be on the UK to prove that EU actions were motivated by an intention to hold the UK within the Protocol, and not merely that the situation had arisen because of "intractable differences" between the parties. It would not be "bad faith" for the EU to adhere to its negotiating position and to pursue the interests of its member states.
With no substantive change in the legal position, the question turns to politics.
In addition to the JII and additional wording in the Political Statement, the EU accepted publication of the UK's unilateral statement on the operation of the backstop.
This indicates a strong political will to secure the WA and to move beyond Brexit day so that negotiations concerning the future relationship between the UK and the EU can begin. While not altering the terms or mechanisms set out in the WA, or limiting the potential duration of the "backstop", both the UK government and the EU seem determined to break through the WA deadlock.
The "meaningful vote"
Given that there is no substantive legal change to the WA, the outcome of the vote scheduled for 12 March is likely to be determined by the degree of concern across the House of Commons at the looming prospect of a "no deal" Brexit on 29 March. Arguably, the principal benefit of the WA would be the transitional or implementation period, holding in place most elements of the current legal position and allowing businesses more time to adjust to Brexit. With only 18 days to go until 29 March, the possibility of a transitional period up to 31 December 2020 must have strong attractions. Whether that is sufficient to overturn January's 230 vote rejection of the WA seems diminishingly likely.