The Irish border question has delivered us the backstop, which has brought Brexit to a complete stop.
Why, though, has this border in Ireland been such a conundrum? The UK won't be the only country with a land border with the EU after Brexit, so why not just copy the model for what happens elsewhere?
Normally, it is fairly simple in these other countries: there is border infrastructure in place, such as customs check points if that country isn't in the Customs Union.
Some borders have electronic checks and controls to monitor the border with very limited need for interference from border guards.
The problem is no other land border with the EU is like the border in Ireland. Nothing even comes close.
Our ITV News team has been across the island of Ireland to find out why it has so far been the Achilles' heel of every attempt to negotiate the UK's timely exit from the EU, seeing off ministers and prime ministers in the process.
We have also spent a day with the Police Service of Northern Ireland to better understand the challenges they face with the prospect of controlling this border.
To start, a reminder about some of the history of this border. It isn't very old - less than a century ago the border was implemented and Northern Ireland was born in 1921.
Ever since, this controversial line of partition has been the source of much of the anger behind the Troubles which saw decades of civil war and more than 3,500 people killed.
Border infrastructure and anyone guarding the border became 'legitimate targets' for dissident Republican terrorists in Northern Ireland.
These groups have for decades demonstrated they are armed and willing to attack anything that even resembles this dividing line of a border.
Though diminished in number, these groups are still capable of attacks against anything that is a symbol of partition and, in their eyes, Britain's presence in the island of Ireland.
The Good Friday Agreement brought a kind of solution and a kind of peace. It enshrines in law the right for people in Northern Ireland to identify as British or Irish, and, crucially, it renders the border invisible.
You can, if you wish, forget it even exists. The problem is Brexit makes the invisible real again. It imposes differences either side of that border which don't exist now.
The man responsible for preparing police in Northern Ireland for Brexit told me No Deal would re-emphasise that controversial border, and create potential for public disorder.
The PSNI's Deputy Chief Constable also says any attempt to police the border would be a backward step for Northern Ireland and put his officers in the firing line.
So worried is he about the risk it would put his officers under, he has told the UK Government he would resist any request to put personnel on the border.
The PSNI has worked hard to build relationships and trust in areas of Northern Ireland where they previously faced hostility.
The Deputy Chief Constable says this has been about community policing away from the border, but No Deal has the capacity to undermine the progress they've all made together.
DCC Stephen Martin also told me he isn't the only one preparing for Brexit in Northern Ireland.
The dissident Republican terrorists are waiting with bated breath to capitalise, hoping any negative impact on the economy will drive people into their arms in a recruitment drive against the British state.
The UK and the EU take this threat seriously.
For this reason, they are both still trying to figure out how they can keep this border open and unchanged, while also trying to implement the changes that Brexit will bring.
How, though, does the UK intend to "Take Back Control" of its borders if no changes can be made to its only land border with the EU after Brexit?
How do you monitor things like immigration and the flow of the goods along this new frontier with the EU?
And similarly, how does the EU intend to protect its new border?
The UK Government says it absolutely will not put new checks on the border.
The PM Boris Johnson says the new changes can be enforced electronically, away from the actual crossing points using technology.
Interestingly, the PSNI team preparing for Brexit say they still have not seen any sign of the technology that’s meant to capable of doing that job - and given their role in this, they might expect to have been informed by now of what it is, where it’ll go, and how it works.
Chief Constable Simon Byrne discusses why electronically policing the border would not work
For a start the dissident Republican terrorists would cut down any cameras as symbols of the 'British border in Ireland.'
If the cameras are sabotaged, they will need protection. Anyone trying to protect them would then be targeted. And so the cycle of violence begins again.
We also joined the PSNI patrolling a border area in Newry. To operate a single check point for standard licence and insurance checks, they brought a small army.
All officers were armed. There were four armoured support vehicles. A stinger trap was in place, ready for anyone trying to evade them - or worse.
Just this week there was a bomb attack near the border which was an attempt to kill police officers.
The PSNI has blamed the Continuity IRA for this one, but it was the fourth attempt on the lives of their officers this year.
The threat is real and they take every precaution.
There's also the complexity of the border itself. It weaves in and out for 310 miles, cutting through rivers and hedges and towns and villages and church yards.
The local inspector told me his officers need to double check where the border even is when they're patrolling. There are no clear markings and his team isn't allowed to cross into the Republic of Ireland if they take a wrong turn or edge too far to one side on a country road. It really is that hard to know where Northern Ireland begins and the Republic of Ireland ends
It also has more than 330 crossing points - many of them rural side roads and a headache to monitor. Compare that with the longest border in the world between America and Canada which only has 119 crossing points and you get a sense of why this is not straightforward.
Inspector Sam Hoey explains the risks involved if Northern Ireland police officers accidentally cross the 'invisible line' into the Republic of Ireland
Police here warn it is not just the terrorists waiting to capitalise: a No Deal creates new criminals who can make money smuggling across the nooks and crannies along the border, taking advantage of any differentiation in taxes and tariffs.
Smuggling was big business when the UK military used to have check points at the border; a No Deal Brexit could see that black market flourish once more.
There are certainly enough locals with the experience and know-how to make money from contraband which has in the past traditionally gone on to fund weapons for the terrorists.
Deputy Chief Constable Stephen Martin on why the PSNI is preparing for possible 'public order issues' in Northern Ireland after Brexit
This border in Ireland is nothing like the Sweden/Norway border or the Switzerland/France border. There really is no comparison.
Whatever the solution to the Irish border question is - and whether there is one that upholds the peace or not remains to be seen - it will be completely unique.
From what we have reported across the border, this needs new thinking and a bespoke design for it to be sustainable.
Time is not on the side of the politicians. And for the police in Northern Ireland, finding that answer really is a matter of life and death.