The Irish backstop. The UK prime minister wants it scrapped, while Dublin and Brussels stand firm.
The system aims to avoid a hard border by staying aligned with some rules of the EU single market post-Brexit. Something UK Conservatives fear keeps the UK tied to Brussels indefinitely.
For politicians in Ireland, the issue runs deeper than trade.
"It's not to best Boris Johnson, or to annoy English people or British public opinion. These things are bottom lines because they are bottom lines. They are bottom lines because we have to protect stability and the peace on this island. I would have thought that that is an objective shared, not contested, but shared by all strands of British political opinion, including Tories," Mary Louise McDonald, Sinn Féin Leader tells Euronews.
During thirty years of conflict— known as the "Troubles" from 1968 to 1998 — the Irish border was militarised. It ended with the signing of the Good Friday agreement. For Irish politicians, a No Deal Brexit threatens the progress.
"Will it create security tensions on the border? Yes it will. Where will that end? God knows. So it's a risk. We're only out of 30 years of conflict...So we've had 20 good years of the economy in the North doing well and the economy of the South doing well. Good relationship with the UK....All these things were positives and now we're into a list of negatives," Bertie Ahern, former Irish Prime Minister tells Euronews.
The weight of history weighs upon the British and Irish and EU leaders to find a way out of the backstop stalemate.