I see a repeated assumption that puzzles me. From another well asked and answered question:
On one hand it is impossible for the UK to leave the Single Market while maintaining an open border with an EU member
The comments and answers point out that both the Irish and the UK do not want a hard border, so why is it impossible to leave the Single Market without one? I presume the EU cannot legally force a country that is not a member to enforce its border with them and it can't force the UK to remain an EU member if it decides on a soft border.
This question also includes the assumption and has an answer titled "They can't" but the reasons it gives aren't things that create impossibilities. For example, it claims:
any Brexit implies stricter borders
Maybe for Ireland/EU but the UK does not have to impose any border restriction. It seems to be assumed, and yet it's been a soft border for almost 100 years:
the border is essentially open, allowing free passage of people since 1923 and of goods since 1993
I'm looking for something of the ilk of a technical/legal reason, not one that is about negotiating stances or consequences or political inconveniences that go "against Brexit". I'll give 2 examples of things I'm not looking for and why because I want this to be a very strict question.
- Negotiating stance: The EU demands it or it refuses to sign a trade deal. That doesn't make it impossible even though the UK could concede to it, the UK could simply ignore it.
- Consequence: (From this answer) The UK would have to accept the free flow of EU citizens over the border. If foreigners can master the Ulster accent well enough to hide illegally then good for them! ;-) But, this also doesn't make it impossible.
It's an assumption so often made and rarely challenged that I'm very curious to find the answer. It appears to be the EU's problem entirely as it would threaten their customs union and that they are the ones that require a hard border after Brexit. However, maybe I missed something obvious or there's an international law I'm unaware of that makes it so?