The membership of Sinn Fein have voted to support policing in Northern Ireland for the first time in the party's history.
The size of the majority was a surprise
BBC NI home affairs correspondent Vincent Kearney reports from the special party conference in Dublin which was attended by more than 2,000 people.
This decision marks a truly historic change of direction for republicans. For the first time in their history, they have declared support for the police and the criminal justice system.
It's a decision that would have been unthinkable just a few years ago, when republicans were still openly hostile to the police and the institutions of law and order, which they regarded as key parts of the British state.
More than 300 police officers were murdered during the Troubles, the majority of them by the IRA.
Members of the judiciary were also killed, courts were bombed and for many years republicans even refused to recognise those courts, dismissing them as illegitimate.
Gerry Adams and Martin McGuinness were branded traitors by republican dissidents
Now all that is set to change. The story of Paul Butler, a Sinn Fein councillor in Lisburn, demonstrates just how a big change it is.
When he was 17-years-old, Butler killed an RUC officer, and he spent 15 years in jail as a result. But he was one of those who spoke in favour of the motion, saying it was time for the party to change its position.
The result of Sunday's vote was never really in much doubt. Gerry Adams was expected to win this vote from the moment he called the special ard fheis (conference).
But the size of the majority was a surprise - the motion to change the party's policy on policing received the support of more than 90% of delegates.
The Sinn Fein leadership had clearly worked hard in recent weeks to convince the doubters that this was the right move.
There were public meetings, attended by thousands across Northern Ireland, but the most important meetings were the ones that took place away from the public gaze - discussions with IRA members, former prisoners, and party members who were eligible to vote.
Once the support of those key elements was secured, the vote was a formality.
But that does not diminish the significance of this move. The word historic is used a lot when talking about Northern Ireland politics, but this is a day when its use is entirely appropriate.
It was also hugely appropriate that one of those public meetings to discuss policing took place in the majestic surroundings of Clonard Monastery in west Belfast last Wednesday night.
Because Clonard is where Sinn Fein first embarked on the journey that brought them to this point.
The Sinn Fein leader told his Clonard audience that he made up his mind on policing just eight weeks ago. One of those at the meeting quipped: "Did he say eight weeks or eight years?"
He may not have made up his mind eight years ago - but he had certainly taken the first steps on the journey.
'Change its policy'
And it was in Clonard that those first steps were taken, when Gerry Adams engaged in secret talks involving John Hume when he was leader of the SDLP and local priests Fr Alex Reid and Fr Gerry Reynolds.
Those talks created the political process that led to the IRA's ceasefire and the talks that led to the Good Friday Agreement, and a new start to policing was part of that agreement.
For a long time Sinn Fein played hardball, as they always do, insisting that support for the police was a bridge too far.
But they said that about taking their seats in Stormont and the IRA decommissioning its weapons.
For those who have watched this process closely, it has for a long time been a question of when, not if, Sinn Fein would change its policy on policing.
That does not mean there is not internal unrest and opposition. There were some dissenting voices during Sunday's debate, with a number of speakers outlining their opposition to the move.
Gerry Adams and Martin McGuinness were branded traitors by republican dissidents as they arrived for the meeting, and at one point Gerry Kelly, the party's spokesman on policing, was interrupted during a live television interview by a protestor who accused him of selling out republicans.
It is an issue that has divided families. I know of one couple from north Armagh who stopped discussing policing at the dinner table - the husband believes Sinn Fein should have signed up to policing years ago, while his wife believes that the day hell freezes over would still be too soon.
That kind of division was illustrated during the debate when Sean McGlinchey, the brother of former INLA leader Dominic, addressed the delegates in support of the motion. Another brother, Paul McGlinchey is to stand against Sinn Fein in the assembly elections on an anti-republican ticket.
But there were no walk-outs, no talk of organised opposition. In fact all the talk, from opponents of the motion as well as those who supported it, was of the need for unity, of the importance of critics staying within the party.
Mr Adams said the vote was truly historic
So what now? Sinn Fein does not at this point support the police - the motion gives its ruling executive, the ard chomhairle, the authority to declare support for the PSNI and the criminal justice system only when the Stormont Assembly is restored and policing and justice powers are transferred to it, or if this doesn't happen, when new arrangements to implement the Good Friday Agreement are in place.
Those conditions may cause a problem for the DUP, which has said it wants to see Sinn Fein implementing its change of policy before it will agree to a restoration of devolution.
What happens when the ard chomhairle announces that it is to endorse the police? At that stage Sinn Fein members will join the policing board and they will be expected to encourage their supporters to view the PSNI as the legitimate upholders of the law.
Will that mean Sinn Fein members and supporters lining up to apply for the right to don the uniform of the PSNI?
That sounds unthinkable, incredible, even ridiculous. But then again for almost a century, so did what happened today.