The U.K.'s exit date from the E.U.
Keeps getting closer, and there's still one major issue threatening a smooth Brexit: the Irish border.
The boundary between the U.K. territory of Northern Ireland and the sovereign state of Ireland is set to become the only land border separating the U.K. from the E.U.
It's also seen decades of violence between pro-U.K. and pro-independence factions in the region during the late 20th Century.
David O'Sullivan is the European Union's Ambassador to the U.S. He told Newsy the E.U.
Was a key part of ending the Northern Ireland conflict, keeping Northern Ireland in the U.K. without cutting it off from the Republic of Ireland.
He told Newsy, "I think many people don't understand the extent to which joint membership of the European Union was instrumental in the peace process.
Since we're both members of the European Union, in practice, there's no border on the island of Ireland, and you live on the island of Ireland as though you're living in one country." There's technically always been an international boundary between Northern Ireland and Ireland.
But since both sides were part of the E.U., neither one really had to force the issue.
Now, Brexit's about to change that.
After Brexit, the U.K. will likely set up trade deals and economic policies that differ from E.U.
That means any goods or people passing across the Irish border will need to be inspected.
But shoring up the border is harder than it sounds.
Ireland's border with Northern Ireland stretches about 310 miles, and officials on both sides figure there's about 208 crossing points on that boundary line.
For comparison, the E.U.'s entire eastern border — spanning nine countries and 3,720 miles — has an estimated 137 crossing points.
SEE MORE: The EU Isn't Budging On Its Brexit Deal With Theresa May So why is the Irish border so much more complex?
Well, it wasn't really designed to be an international boundary.
O'Sullivan said, "Americans will understand when I say these are county boundaries, they're administrative boundaries, they were never, you know, there's not a river, there isn't a mountain range.
And this border crisscrosses the countryside, some people have farms, which are on both sides of the border, the border goes through people's living rooms." "You can drive, you know, five kilometers in one direction, turn right, and you cross the border one way, a kilometer later you turn left and you cross the border another way.
So it's almost impossible to hermetically seal this border." A porous border could make Northern Ireland a target for smugglers looking to skirt onerous trade policies.
O'Sullivan worries that could re-empower some of the groups behind the Northern Ireland conflict.
O'Sullivan said, "Some of the people perhaps best equipped to exploit that situation could be the former paramilitaries, the former terrorists, now retired, but who might, you know, go into business.
I think the fear is that the complexity of the border, the risk of criminal activity which could then take a dangerous turn." Negotiators haven't figured out a way around these problems yet.
Instead, they've reached a fallback plan which requires the U.K. to stay aligned with the E.U.
Unless a more permanent solution is worked out.
But opposition from U.K. Parliament over that plan could sink the entire deal and push both sides towards a chaotic no-deal Brexit.