Editor's Note: (Nic Robertson is CNN's international diplomatic editor. The opinions in this article belong to the author. )
(CNN) Northern Ireland is becoming Brexit's square peg that refuses to squeeze into a round hole.
Nearly 18 months after Britain's decision to leave the European Union, it is becoming increasingly clear that this single issue could become the sticking point in negotiations -- one that ultimately leads to no post-Brexit deal.
More startling, however, is that the problem is not new. Now, just days before this core issue of how to handle what will become the UK's only land border with the EU requires clarity and detail, Britain appears no closer to a solution.
Of the three tasks the EU set the UK to tackle before agreeing to begin talks about a future relationship -- financial commitments or the so called "Brexit bill"; the rights of EU citizen workers; and the Irish border -- the other two now pale in comparison.
On the 14th and 15th of December, the leaders of EU member states will decide if the UK has done enough to move to the next phase of talks -- the future relationship between the EU and the UK. It's the second time this deadline has come: previously, the EU said insufficient progress had been made on the three issues.
Failure this time could leave the UK struggling to cut a deal before they leave the EU on March 29, 2019, at huge cost to the British economy.
Since the last deadline was missed, extra money has been found to help settle the UK's commitments to the EU and new language to pay bills "when they fall due" will be accompany the offer.
Rights for EU citizens living in the UK have also been clarified closer to the EU's liking. But for some reason tackling Irish border has been left till last.
Relations between Britain and Ireland have bedeviled generations of British leaders. Now Theresa May, the UK's Prime Minister, is left holding the poisoned political chalice that Brexit has unearthed.
The enormous difficulties concerning the practical reality of any border between the Republic of Ireland (an EU member state) and Northern Ireland (which will leave the EU along with the rest of the UK) have been staring anyone who cared to look in the face.
Westminster, not for the first time in recent years, seems to have woefully underestimated sentiment outside the UK's capital, as well as the EU's resolve to back its member states.
Some of the problems were entirely predictable.
Theresa May says Brexit means leaving both the EU's Customs Union and Single Market. That in turn means the free flow of goods and people across Europe's borders with the UK will fundamentally change.
As the Republic of Ireland is part of the EU and shares a 310-mile land border with the UK Province of Northern Ireland, the future operation of that border is now in question.
Since the Good Friday Peace Agreement 20 years ago ended decades of sectarian violence that led to more than 3,500 deaths, the border has been open. British Military border posts and watch towers were taken down while border checks and blocks were removed. As a result, trade between the north and south improved.
Today an estimated $1.5 billion dollars worth of business flows back and forth between Ireland and the UK each week.
Farmers cross over near constantly to buy fuel and feed or sell livestock. Villagers on both sides rely on doctors and hospitals over the border. Thousands of trucks trundle daily across an estimated 300 to 400 border crossings.
There is much at stake and the Irish government will likely hold the British government responsible for any knock-on effect Brexit has on Ireland's economy.
The Irish government, backed by the EU, says Brexit must not lead to a hard border between the north and south of Ireland and not destabilize Northern Ireland's peace process.
Logic dictates that Brexit means either some border adjustments, or it is left wide open and border controls are implemented at mainland UK ports and airports.
The problem with the second scenario is Theresa May's government is propped up by politicians who won't stand for any hint that Northern Ireland is a lesser part of the UK or is drifting towards a tighter partnership with Ireland.
When May called her disastrous snap elections last summer -- slashing her government's majority -- she was forced to turned to Northern Ireland's Democratic Unionist Party for support.
They are the most hard line pro-Unionist party in Northern Ireland whose refusal to compromise is legendary. May cannot make a deal that upsets them. They would regard anything that makes Northern Ireland's status different to the rest of the UK as unacceptable.
Making matters even more complicated is the DUP's political nemesis, Sinn Fein. They are the most powerful pro-United Ireland party in the North and are unsurprisingly pushing for the polar opposite of the DUP.
Sinn Fein is demanding May declare a "Designated Special Status for the north of Ireland within the EU."
May has only one option: keep the land border open and controlled at the same time. The problem is that this is probably impossible.
Ireland's Taoiseach Leo Varadkar is demanding a commitment from May in writing guaranteeing there will be no hard border, otherwise he'll block the Brexit talks.
Just days before May meets with European Commission President Jean Claude Junker -- where she hopes to get a nod about progressing to post-Brexit trade relations -- the issue is far from resolved.
On Wednesday, referring to the Irish border issue, Junior Brexit Minister, Robin Walker conceded: "we would like to a reach an agreement with the European Union in which there is no need for queues on either side of the border."
Walker added that the UK government wants to ensure there is "no physical infrastructure at the border ... the government absolutely recognizes that we want to ensure that there is no return to those borders of the past."
In the past, what the British have regarded as sufficient has been politely welcomed by the EU, but rejected as lacking necessary detail.
Having cajoled May into conceding so much on the other two issues it would be a surprising negotiating tactic of the EU to give her a free pass on the border issue, knowing that with more pressure and more time they and the Irish Taoiseach will likely get more of what they want.
When it comes to Ireland, London might just be beginning to get the message: The UK no longer has the power to dictate terms, least of all to a former colony with which it has spilt almost a millennia of bad blood. And certainly not when that former colony has the EU's backing.