If dissident republicans were responsible for the missile attack on the headquarters of MI6, in conventional republican terms they have scored a spectacular success. Mr Blair's problems were wiped off the front pages here in London; rail services were severely disrupted, compounding a traffic snarl-up which kept Queen Elizabeth late for her appointment at the Commonwealth parliamentary gathering at Westminster; and there were no fatalities.
The instant wisdom is that the republicans had learned the lessons of Omagh, striking a prestige target while minimising the risk of death.
Police and security chiefs would instantly and correctly counter that people planting or firing bombs can make no such calculation - that destruction without death is a matter of luck rather than terrorist design.
Even before the MI6 attack it was clear that dissident republicans were intent on restoring terror to Britain's streets, and had developed the capacity to do so. In his recent address to the British Irish Association, the Northern Ireland Secretary, Mr Peter Mandelson, left no doubt about the British government's assessment of the seriousness of this republican threat.
Some listening to him wondered, indeed, if he was contemplating a return to internment without trial. But if received Irish wisdom, and all past experience, would rule out that possibility, there has been, too, a sense of impatience that draconian legislation enacted in the Republic after the Omagh bombing has not had conspicuous effect.
In times past - when the Provisional IRA was bombing and killing on a large scale - the more measured response of government in both capitals was to stress the primacy of politics. They will do so again. And they will do so, moreover, in the dramatically changed circumstances of Northern Ireland, where Sinn Fein now sits in government alongside the Ulster Unionists.
The new dispensation is still a delicate creation. Indeed, the attack on MI6 came on the eve of a by-election which itself - regardless of the result - seems certain to add to the stress levels within the Ulster Unionist Party, and to further complicate that party's dealings with Sinn Fein, the SDLP and the two governments.
Even assuming the outcome which would be most benign from Mr David Trimble's perspective, the ongoing debate about the Patten proposals for a new beginning to policing in Northern Ireland has the potential to derail the Belfast Agreement. Certainly pro-agreement unionists fear the Executive itself would prove unsustainable should the SDLP finally decide that the Police Bill falls so far short of Patten that it cannot take its seats on the new Policing Board.
There are potent risks in all of this to which both governments are highly sensitive. Nobody doubts it could still all go horribly wrong. Yet there is the comforting belief, too, that the parties remain committed to the agreement and to making it work. On that working assumption hangs the belief that the difficulties over policing can be navigated.
The prize is almost unimaginable, notwithstanding the changes that have already occurred in the past two years - that nationalist leaders should declare the police service acceptable to their communities and a fit and proper career choice for young people, nationalist and republican, within them.
And if it is attained it is surely hard to see how the "Real IRA" or other republican dissidents could hope to sustain a long-term campaign.
As with the loyalist feud, the danger is ever present that one atrocity would prove enough to lure other paramilitaries back into the conflict zone. Events can quickly run out of control, a calculation the republican dissidents presumably make in hope.
Hence there will be a renewed determination by London and Dublin to push ahead and resolve the political problems still threatening the process.
Even small numbers of determined people will retain the capacity to inflict terror. But that process has yielded the prospect of a settled dispensation in Northern Ireland, underwritten by the votes of the people, North and South, in dual referendum.
The logic of it is not merely that the two governments have the authority to bear down on any who would threaten to oppose it other than by peaceful means, but that they will be able to do so with the support of all those - Sinn Fein included - who now share responsibility for the government of Northern Ireland.