With the near total collapse of power-sharing in Northern Ireland, it would be well to remember why such an unstable system of government was established in the first place. It’s this: neither the nationalist or unionist community trusts the other to govern alone.

So when first minister Peter Robinson’s logic led him to pulling his party out of the Northern Ireland executive, he couldn’t quite do it. He had reached that point because of an accumulation of crises.

The first was the deadlock in efforts to agree to pass on Tory welfare cuts. All parties thought they had hammered out a compromise last Christmas, enabling subsidies to welfare recipients to cushion the blow. And when Sinn Féin withdrew assent to this compromise, saying that the letter of it had been changed, Northern Ireland was left with a budget that will only stretch to October.

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That whole schmozzle has contributed to a growing sense in Northern Ireland that devolution doesn’t work and that we would be well rid of it. We were on our way to a possible breakdown even before the chief constable announced last week that IRA members had participated in the murder of Kevin McGuigan, one of their own former assassins who had fallen out with them. They suspected McGuigan had killed another republican, Gerard Davison, and was thinking of shooting a few more.

Even that might not have wrecked the whole system. But Unionism was freaked by the disclosure that the IRA still exists with a command structure and access to guns.

The Sinn Féin deputy first minister, Martin McGuinness, was quick to dismiss the killers as criminals, providing them with no cover. And the chief constable softened the shock by saying that there was no evidence the killing of McGuigan had been sanctioned by the IRA: he firmly believes Sinn Féin and the IRA are committed to peaceful politics.

Which might have calmed nerves, but this was the moment for the smaller unionist party, the Ulster Unionists, to take an initiative. Its leader, Mike Nesbitt, pulled his minister out of the executive. In the power-sharing allocation of seats round the executive table, the UUs had only one ministry, but that put a huge squeeze on Robinson to follow.

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He resisted that with half-measures, such as boycotting the executive, but then the police moved again. They arrested three top republicans, all with reputations for past careers in the IRA. One of them was Bobby Storey, or Bob Mor as Gerry Adams calls him, a former head of IRA intelligence and northern chairman of Sinn Féin.

Storey is a hard and scary man. He served an 18-year sentence for murder, with a top-up of another seven for helping to lead the major breakout from the Maze prison in 1982.

Robinson has dug in further than he might have done had the Ulster Unionists not threatened to outflank him

Storey was released from custody last night. But intelligence had led the police to suspect him of the murder, and that was enough to force Robinson to push for an adjournment of the assembly and to pull his ministers for want of that. He would then have preferred the secretary of state, Theresa Villiers, to suspend the assembly, but she refused.

Now Robinson has got his ministers to resign, leaving only finance minister Arlene Foster in place to guard the money bags.

He has dug in further than he might have done had the Ulster Unionists not threatened to outflank him as the authentic voice of wary and disillusioned unionism. And he has created a mess. Now he needs to trust that David Cameron understands it would be simply impossible to let nationalists govern Northern Ireland almost alone.

If Cameron grasps that fact, he will suspend devolution now. If he doesn’t he will perhaps let Villiers call an election, merely to exasperate the disillusioned and defer the inevitable crash of a bankrupt and riven assembly.