Gerry Adams, the president of Sinn Fein, was arrested by the Police Service of Northern Ireland (PSNI) on April 30.

He was held for four days in Antrim police station, in a serious crime interrogation unit. The 65-year-old Adams was questioned, reportedly for up to 17 hours a day, on the 1972 murder of Jean McConville by a unit of the Provisional Irish Republican Army (IRA).

Adams attended the police station voluntarily, having for years denied any involvement in the killing. On his release on May 4, without charge, he announced, “I want to make it clear that I support the PSNI.” Nevertheless, his arrest testifies to pressures building up around the Stormont administration in Northern Ireland.

Adams, a republican since the late 1960s who became leader of Sinn Fein, the political wing of the now disbanded IRA, in 1983, is one of the most well-known figures in world politics. The suggestion that any aspect of his activity is unknown to the British or Northern Ireland governments is ridiculous. He has been the subject of the most intensive observation by political and intelligence agencies of the British state since he was whisked from internment to talks with the British government in 1972. He was saved on at least one occasion from a loyalist assassination attempt by the intervention of British forces, fearing a less predictable Sinn Fein leadership.

Adams has repaid the confidence placed in him. He was one of the key architects of the 1998 Good Friday Agreement that ended the IRA’s conflict with the British government and brought Sinn Fein into the British government in Northern Ireland. Arresting Adams places a question mark against the entire political framework used to open the North of Ireland for investment based on tax breaks and cheap labour, while dismantling the large British military apparatus.

Acknowledging as much, Northern Ireland deputy first minister and Adams’s lifelong ally Martin McGuinness angrily denounced the arrest as pointing to a “dark side” within the PSNI that was “maliciously and vehemently hostile to the peace process.”

McGuinness, who only a few weeks ago was hobnobbing with the Irish president and toasting the British queen, when asked if Sinn Fein would withdraw support for the PSNI if Adams was charged, warned it would be a “much more serious situation than the one that we face today.”

Extraordinarily, the British government was almost silent in the face of Adams’s arrest. Prime Minister David Cameron baldly told the press, “We have an independent judicial system...and there has been absolutely no political interference in this issue.”

Northern Ireland Secretary of State Theresa Villiers, commenting on the sharpest crisis since she took office in 2012, said she had only been told of the imminent arrest last Monday.

Two explanations for these implausible comments suggest themselves. Both point to a deepening crisis within the British state apparatus.

The first is that the arrest was instigated in Downing Street at the behest of the most right-wing elements around the British military. The Cameron administration is certainly increasingly hostage to frothing anti-European Union (EU), anti-immigrant elements within the British establishment.

Jean McConville was a mother of 10 who lived in the predominantly Catholic and deeply impoverished Divis flats housing complex in Belfast. She was accused of being a British informer by the local IRA unit, abducted, tortured and shot in the back of the head. Her body was not found until 2003. The case, a particularly tragic and emotive one, is being exploited to distract attention from two of the most notorious massacres of the “Troubles” in which, unlike the McConville case, British forces are directly implicated.

Days before Adams’s arrest, Villiers announced that no public inquiries would be held into the Ballymurphy Massacre of 1971, when 11 Belfast civilians were killed by the British Army’s Parachute Regiment, during the Operation Demetrius campaign of mass internment. Villiers told relatives of those gunned down, “In my view, the balance of public interest does not favour establishing an independent review.”

The same day, Villiers rejected an inquiry into the 1978 firebombing by the IRA of the La Mon hotel. Aimed at members of the Royal Ulster Constabulary, the botched attack instead killed 12 members of the Irish Collie Club. The La Mon case has been the subject of allegations that some of the participants in the IRA attack were British agents.

Adams’s arrest ensured neither massacre made the headlines. Certainly one component of the arrest is the warning it sends to Sinn Fein’s leaders against pushing too hard regarding the role of British forces.

Alternatively, if, as suggested by McGuinness, the arrest was only belatedly signed off on by Downing Street, and initiated by hard-line unionist elements within the PSNI, then it points to an increasing crisis within Ulster unionism as an expression of a deepening and general crisis of the entire British state apparatus.

Political tensions in Northern Ireland have been sharply exacerbated by the upcoming referendum for Scottish independence. Former Northern Ireland first minister David Trimble of the Ulster Unionist Party told the BBC, “The only thing I see on the horizon that could cause a serious problem...is the referendum in Scotland”.

Trimble’s and unionism’s profound concern is that in the uncharted chaos that would accompany Scotland’s departure from the UK in the event of a yes vote for Scottish independence, the position of Northern Ireland would become untenable. Northern Ireland could rapidly confront a situation where its nearest neighbours to the South and East were both outside the United Kingdom and offering far lower corporation tax rates. Sinn Fein’s call for a referendum on Irish unity would rapidly become irresistible.

Little wonder then that Sinn Fein, advocates of a united capitalist Ireland, should become the target for frenzied assaults from unionists. Local and European elections are due to be held May 22. Sinn Fein is the only party that operates on both sides of border. A pro-business party of the aspiring Catholic upper middle class, but which peddles a certain amount of left rhetoric, Sinn Fein is expected to do well.

For their part, political commentators in the Republic of Ireland seized on Adams’s arrest to call for his removal as Sinn Fein leader and his replacement by Mary Lou McDonald. McDonald, a generation younger, is considered a more pliable figure, a candidate for a coalition government. This is an urgent consideration as the Fine Gael and Labour coalition is now as hated as its Fianna Fail and Green Party predecessor.

The entire episode shows the unstable character of the political setup in Northern Ireland.

For all sections of workers, Protestant and Catholic, North and South, neither a continuation of the sectarian arrangements in the North, still less a return to open conflict, nor a unified capitalist Ireland offers anything but deepening inequality and civil strife. Only through an independent struggle for workers’ governments in Ireland, Britain and Europe can the complex and deeply rooted historical and religious divisions, the legacy of centuries of British rule in Ireland, be overcome on a progressive and truly democratic basis.