This is a moment to hold one's breath. The last murderous barracks assault on British soldiers in Northern Ireland took place a dozen years ago at Thiepval. Then it was not unexpected; this time the assault has a terrible quality of nightmarish unreality. But the Northern Ireland peace process has proved surprisingly durable. It even survived the mass slaughter at Omagh in 1998. The key now is the response of the Sinn Féin leadership.
Thus far that leadership has felt able to condemn the attacks of dissident republicans. But there are two fiendish complications this time. First, the target is the British Army not the local police force. Ideologically speaking the dissidents have now challenged Mr Adams in a way the murder of fellow Irishmen would not have done. This is a decisive moment in the history of the “Brits Out” mentality in Northern Ireland.
That is why Mr Adams is careful to denounce the murders as counter- productive in the struggle for Irish unity. But he has also said that they were wrong and more important that he supports the police in the apprehension of the killers.
This comes at a moment when the Chief Constable's decision to employ Special Forces soldiers in the Province presents Mr Adams with his second difficulty. Sir Hugh Orde will undoubtedly argue that his decision was justified by the seriousness of the security crisis now so amply demonstrated. Mr Adams initially responded that the decision meant the Chief Constable risked losing the support of the majority of republicans.
This row over the role of British intelligence in the province goes back to ambiguities in the St Andrews Agreement of 2006. It has profound roots in the psychology of republicanism and the legacy of one of Mr Blair's creative formulations to move the peace process along. It will not be easily sorted out.
The crisis both validates the approach of the Secretary of State Sean Woodward and demonstrates its limitations. Mr Woodward has been convinced that Sinn Fein's strength is directly correlated to progress on devolution in general and devolution of policing and justice in particular. This explains his rather strange decision to railroad the Northern Ireland Bill through the House of Commons last week, despite sustained protests by MPs from all parties. They ask why, when the actual devolution of policing and justice was not exactly imminent - one DUP minister said it could be a matter of several years - the Government acted in such haste. But for Mr Woodward it was a no-brainer. As the DUP agreed the devolution model, while retaining its veto on timing, the Government felt it was possible to offer something to show that Sinn Féin was still making progress. The very indignation of MPs generated by the indecent haste was almost a selling point in itself. Sinn Féin still had visible leverage.
The trouble is, that for all the Secretary of State's solicitude, this is small beer. The crisis of Sinn Féin is far wider and deeper and is due to developments about which the British government can do little. The unexpected electoral humiliation of the party in the 2007 general election in the Republic has derailed Mr Adams's all-Ireland political strategy. Even now, as the Irish Republic enters deep recession, it is the Irish Labour Party rather than Sinn Féin that appears to be picking up the protest vote. In the North the DUP, SDLP and many pundits hail the DUP's success in blocking Sinn Féin projects in the Assembly. The British Government might privately wish there was less of this triumphalism but, having failed to protect David Trimble's more conciliatory leadership of Unionism, it is in no position to complain. The consequence of all this is that to some minority elements of nationalist youth in Northern Ireland the Sinn Féin leadership looks boringly respectable and even a little moth-eaten. The electricity and excitement that it once generated now seem to be a thing of the past.
It is not that Sinn Féin is likely to lose out to the SDLP but more a question of its ability to mobilise. There is even some early indication that nationalist/republican electoral turnout is no longer as vibrant as it once was.
The dissidents themselves remain small in number. Their potential influence lies in their ability to capsize the arrangements that have been reached between larger groups. Already they have succeeded in bringing about the cancellation of the American trip of the First Minister, Peter Robinson, and the Deputy First Minister, Martin McGuinness, to Washington for the St Patrick Day's Jamboree. There is much that is bogus and unappealing about this event. But it is at least an occasion when the political leadership of Northern Ireland - unionist and republican - presents a more or less united face to the world.
For all its difficulties, however, Sinn Fein are not passive players in this crisis. This atrocity in the Protestant heartland of Antrim will shake the Unionist community but it will not destroy support for the current settlement. What would destroy it would be a sense that Sinn Féin did not fully support policing in the Province and instead sought to garner political advantages from the crisis.
The republican leadership's words on the issue will be scrutinised more rigorously over the next few days than those of any shame-faced banker before the Treasury Select Committee. Mr Adams is well aware that the Irish Government is totally absorbed with its own economic crisis and is not available to exert energy and pressure on his behalf in the way it once did. He is also well aware that the British Government has reached the end of the road in terms of the concessions it might meaningfully offer. This gives him a strong incentive to protect his one great triumph, the substantial slice of local power that Sinn Féin enjoys in the local assembly.
Lord Bew, an independent crossbench peer and Professor of Irish Politics at Queen's University, Belfast.