The twin towers attack changed politics across the globe. Suddenly, one man's terrorist was no longer considered another man's freedom-fighter anywhere in the developed world.
The well springs of popular support for ethnic insurrections and underground armies dried up almost overnight.
The effect on our little patch here in Northern Ireland can be traced in IRA statements issued on either side of the September 11, 2001 attack.
On August 14 - less than a month before al-Qaida struck - the IRA ended co-operation with the Independent International Commission on Decommissioning (IICD) and withdrew a plan it had submitted to 'put arms completely and verifiably beyond use'.
The IRA blamed the unionists and the British Government for failing to fulfil their commitments.
Republicans calculated - probably correctly at that point - that they could afford to string out the arms issue and, perhaps, avoid it altogether.
By the time the next statement came, on September 20, their calculation had changed in the face of feedback from US supporters as well as the Bush White House.
The statement opened with sympathy to the people of the US and 'especially the families and friends of the victims of the deplorable attacks'.
The IRA still complained of unionist and British bad faith, but said that it was going to resume contact with the IICD and move towards a 'comprehensive resolution' of the arms issue, anyway.
The US continued to pile on the pressure. Visas for Sinn Fein representatives felt to have an IRA record were now only issued when the Bush administration wanted to meet them to discuss decommissioning.
A month later, the IRA put its weapons in secure dumps which could be inspected by Finnish and South African dignitaries.
In April 2002, the IRA was still complaining that other parties were not fulfilling their obligations in the peace process, but was nevertheless decommissioning its weapons.
The possession of arms and the capacity to use them had once been the key to extracting concessions from the US, Irish and British governments. Now it was a liability.
Republicans were getting rid of their weapons as quickly as they could without causing a split.
It didn't matter if others responded appropriately, decommissioning was something that needed to happen. The process was completed in September 2005 - four years to the month after 9/11.
Things may have moved in this direction anyway, but the twin tower attack created impetus.
It also enabled the Adams/ McGuinness republican leadership to convince doubters that the peace process offered the best way forward and that the option of resuming the campaign should be formally abandoned.
It worked out well for Sinn Fein, which has grown exponentially north and south after renouncing support for violence - but the organisation has changed.
This weekend's ard fheis in Belfast has a long section on Irish unity, but now that goal is to be achieved by co-operation and popular consent for a new Irish constitution.
The option of demanding British withdrawal and defending the right of the Irish people to bear arms disappeared on 9/11.