Jacob Rees-Mogg has an easy solution for the border question complicating UK withdrawal negotiations with the EU. Following a meeting with EU chief negotiator Michel Barnier in Brussels this week, the pure-Brexit Conservative MP said: “the answer on the Irish border is simply not to put an Irish border in.”

If the EU wants to put up a border, so the argument runs, that is up to them. UK Prime Minister May rebuked this argument in her Belfast speech in July: “We can’t solve it on our own, but nor can we wash our hands of any responsibility for it.”

But what would happen if the UK left the EU without a withdrawal agreement? Mr Rees-Mogg has previously claimed that the UK would not have to put up a border in the event of a no deal Brexit, whereby the UK would trade mainly based on World Trade Organisation (WTO) rules.

However, Pascal Lamy, former head of the WTO, told RTÉ in July that the idea there would be no border on the island of Ireland in a no deal Brexit is “pie in the sky.” Lamy added that, outside of the EU, there are no borders anywhere in the world without any physical infrastructure.

Even though WTO practice requires its members to carry out customs checks, no one knows what London would do in a no deal scenario, partly because of febrile Brexit politics at Westminster. If the UK did not introduce customs checks, other WTO members could file a case against the UK for violating the global trading system. Resolving such disputes would likely take a long time, but hardly the international law-flouting image the newly “Global Britain” would want to project.

What would Dublin do? The Irish government says that it does not intend to put up a border in the event of a no deal Brexit. It also expects the same of the UK, based on the political commitment in the backstop agreement last December, even though that would carry no legal weight if there were no UK-EU withdrawal agreement.

That sounds similar to the Rees-Mogg line, but a lot would depend on how acrimonious a no deal Brexit would be. In that scenario, the EU would classify the UK as an outsider (a “third country” in the jargon), which would automatically create legal constraints for EU-UK cooperation.

Moreover, there would be little political incentive for the EU to carry on with frictionless trade with the UK, as no deal would mean London had reneged on concluding the withdrawal agreement, including the border backstop, citizens’ rights, and the ‘Brexit bill’.

What might be possible is that the EU and UK would introduce customs checks, but neither Ireland nor the UK would do so along the land border, at least for a time. Instead, they could both carry out checks mainly at sea ports. So East-West rather than North-South.

That might work de facto for a short time, even if not entirely de jure. But it would be an ad hoc solution, that essentially depended on a no deal situation not lasting very long, say a few weeks. This is probably what Dublin hopes, that a land border is de facto not needed.

But if a no deal situation lasted longer, say a few months or more, the intellectual flaw in Ireland’s (and Mr. Rees-Mogg’s) position may be exposed. The flaw is that if large quantities of prohibited goods come into the EU single market via Northern Ireland to Ireland, Brussels would likely have to pressure Dublin to put up a customs land border.

Dublin might prefer to wait for London to do so first, for political blame reasons, but the result would be the same. In other words, a no deal Brexit is potentially a nightmare scenario for Ireland, for lots of reasons, but particularly as it could lead to a rift between the Irish government and its EU partners – if Brussels insisted on land border checks and Dublin refused.

This would be the worst of all worlds for Irish diplomacy, as its position has depended on the impressive solidarity shown by the other 26 EU governments towards Ireland. In addition, Dublin hardly wants a rift with Brussels over implementing EU and international rule of law.

However, the Irish government is unlikely to change its public line. This is partly for domestic political reasons, partly because Dublin does not want to give any impression that a London U-turn on the December backstop agreement is acceptable.

In a drawn out no deal scenario, the Irish government would probably – very reluctantly – have to follow EU and WTO procedures and introduce customs checks. That would be a very difficult domestic sell for Dublin.

But it would be in Ireland’s strategic interest to avoid a rift with the other 26 EU governments. Ireland could contrast itself to a no deal Brexit-ed UK by sticking to EU rules, or at least be seen to be trying to stick to the rules – even if they were truly unpalatable.

The hope is that a no deal Brexit scenario would not last long. However, the EU would still insist on concluding the withdrawal agreement, including the border backstop, before starting formal trade talks with the UK. The reality, therefore, is that if the UK crashed out of the EU with no withdrawal agreement, it would have to return to the table eventually if it wanted a trade deal, from a weaker negotiating position.

Daniel Keohane is Senior Researcher at the Centre for Security Studies at ETH Zürich. This piece originally appeared in the Belfast Newsletter.