The answer found was the "Northern Ireland Protocol", an adjunct to both sides' Brexit divorce deal that sought to respect the Belfast/Good Friday Agreement by ensuring no checks on goods across the border in Ireland.
Why no checks?
The UK and EU agreed that despite Brexit, a vital plank of the Good Friday Agreement had to be preserved -- the elimination of physical border infrastructure on the island of Ireland.
During the three-decade "Troubles" in Northern Ireland, such infrastructure and the associated British Army presence became a hated symbol for nationalists who want to reunify the island.
But checks had to happen somewhere -- so the UK agreed a de facto trade border down the Irish Sea to ensure goods coming from England, Scotland and Wales did not enter the EU free market unchecked via Northern Ireland.
That has infuriated pro-UK unionists in the territory, although a majority of members of its newly elected assembly led by the nationalist party Sinn Fein back the protocol.
What's in the protocol?
To keep the border open, Northern Ireland effectively remained in the EU's single market.
That meant it had to stay in line with certain EU rules on tax and product standards, unlike the rest of the UK.
Supporters say that gives Northern Ireland the best of both worlds, with access also to the UK's single market.
But unionists fear a fraying of their bonds with the rest of Britain and are refusing to rejoin a post-election power-sharing government in Belfast.
Siding with them, the UK accuses Brussels of applying the protocol too zealously and says the deal must be renegotiated.
What is the UK proposing?
London has announced plans to drastically overhaul the protocol unilaterally through legislation, in an apparent bid to end the political paralysis in Northern Ireland and force the EU into concessions.
The UK has never fully implemented the protocol, arguing that many of the checks are unnecessary and that Northern Ireland could ill afford more red tape on top of the economist costs of the Covid pandemic.
Under the new proposals, it intends to create a "green channel" for British traders to send goods to Northern Ireland without making any customs declaration to the EU.
Brussels would have access to more real-time UK data on the flow of goods, and only businesses intending to trade into the single market via Ireland would be required to make declarations.
The EU would need to trust the UK to monitor the flow, with London vowing "robust penalties" for any companies seeking to abuse the system.
The UK plan would also harmonise tax policy between Great Britain and Northern Ireland. And it would seek to end oversight of the protocol by the European Court of Justice.
However, London has held off introducing the legislation, saying it will give ongoing talks with the EU several more weeks.
What does the EU say?
The EU says Johnson knew full well what he signed up for, and is trying to wriggle out of the real-world consequences of his government's hardline vision for Brexit.
For Brussels, the paramount need alongside protecting the Good Friday Agreement is keeping shoddy, unsafe or untaxed goods out of its single market via the Republic of Ireland.
It has made concessions, including proposing fewer checks on goods heading east-west from Great Britain to Northern Ireland, and has already suspended checks on UK medicines shipped to the territory.
But European leaders, including Ireland's, have insisted the terms of the protocol will not be renegotiated, and are warning of a potential trade war if the UK walks away.
What about the US?
The United States is watching closely -- it helped broker the Good Friday Agreement and is one of its guarantors.
The administration of Irish-American President Joe Biden has warned that London can forget about a post-Brexit trade deal if its actions in Northern Ireland threaten the still-fragile peace.