Consider the following sequence of events:
- In March, the UK fails to reach an agreement with the EU and exits with no deal.
- The UK leaves the British side of the Irish border open, to comply with the Good Friday Agreement (and to comply with the UK’s repeated and strenuous assurances that there would be no hard border). No customs personnel are placed there, there are no immigration checks, etc.
- Ireland begins the process of placing customs and immigration along the Irish side of the border, to comply with EU customs laws.
- The UK accuses Ireland of violating Good Friday, and demands that Ireland keep the border open.
- Ireland is thus forced to either violate Good Friday or leave the EU customs union.
Does anything specifically prevent the UK from doing this? If not, why is Theresa May not using the threat of this outcome as leverage against the EU? Based on the failure of the Chequers deal, it’s obvious she is in dire need of some kind of leverage. Of course, such a threat need not be explicit; she could just as easily say something like this in a public statement:
Although [the latest round of negotiation] was discouraging, the people of Ireland and Northern Ireland need not worry. The UK is fully committed to an open border, even in the unlikely event we do not agree on a deal with the EUC.
We trust that Ireland shares our commitment.
(Later, when journalists start asking questions about the boldfaced sentence, state that it is not UK policy to comment on future actions of other countries and that the journalists must consult Ireland.)
Presumably, the UK would eventually want to negotiate a trade deal with someone other than the EU, and an open border with the EU would make that difficult or impossible, but that’s not nearly as immediate a problem as (5). The question is whether the UK has already made some agreement that prevents this state of affairs, not whether the UK might have a hypothetical future reason to do so.