Yesterday's bomb, which came without a warning, and which security sources believe was the work of dissident republicans, was a wake-up call. As the power-sharing executive met in Belfast for the first time since devolution was restored, it was a sharp reminder against complacency in Britain.
Attacks in Northern Ireland, such as that on an army watchtower in south Armagh last week - the fifth attempted hit on a security force base in three months - garner little media attention. But shutting a bridge on an arterial route into London results in headlines and panic.
Dissident republicans were sowing the seeds of doubt, proving they can still mount attacks, even in England. They aimed to leave people pondering what the group, which killed 29 people in Omagh, Co Tyrone in August 1998, could achieve if it really tried.
It was, too, a warning to security forces in the Irish Republic, who give the impression they have the dissidents under control. Garda ar rested 15 alleged members of the Real IRA last week, including the outfit's 50-year-old leader, after raiding a bomb factory in Dublin.
But yesterday's attack was also designed to embarrass mainstream republicans - the dissidents' favourite trick at critical points in the peace process. They bombed Markethill, Co Armagh, on the day Sinn Fein joined all-party talks in September 1997. The RUC has intercepted several devices at key moments since.
While the former IRA chief of staff Martin McGuinness was attending yesterday's ex ecutive, Gerry Kelly, an ex-IRA hardliner turned leading Sinn Fein figure, was in London to address diplomats. Some find the distinctions between the IRA and dissidents beyond them, and yesterday's bomb made Mr Kelly's task all the more difficult.
According to one security source, Hammersmith Bridge was selected particularly to infuriate mainstream republicans. The IRA failed twice to bomb it, most recently four years ago. To target it now was to highlight both IRA ineptitude and Sinn Fein's sell out.
Security sources on both sides of the Irish border spend little time drawing distinctions between the Real IRA, now three years old, Continuity IRA, dating from the Sinn Fein split of 1986, and dissident elements in the Irish National Liberation Army. They believe they switch flags of convenience as it suits.
But there is no doubt the impetus to dissident republicanism came when the IRA's quartermaster quit the IRA with his partner and a dozen others in the autumn of 1997. They were opposed to Sinn Fein's entry to the talks, and reject the Good Friday agreement which followed in April 1998.
The Real IRA is the largest in a coalition of dissidents, which, security sources estimate, numbers 120 people. But it is unclear how many are either active or potential terrorists.
Security sources are detecting an increase in the shift to the dissidents from the IRA since its offer to put arms beyond use and to allow weapons inspectors to see some of its dumps. However, they say it is hard to quantify.
It is believed that IRA units in hardline areas such as south Armagh and east Tyrone may have secured guarantees from the leadership that their arms dumps will be excluded from any inspections. The former ANC secretary general Cyril Ramaphosa and ex-Finnish president Martti Ahtisaari are expected to be taken to the first dumps this month.
A recent BBC Northern Ireland poll underlined the splits within republicanism. No fewer than 22.5% of Sinn Fein voters believed the initiative was tantamount to surrender.
That may sound an easily manageable figure, but strip away the middle-class Catholic support Sinn Fein is now en joying because of the IRA ceasefire, and it is clear that the figure rises dramatically among the party's natural constituency. Sinn Fein has only moved forward with difficult policies when the leadership has 90% backing.
The Real IRA has, officially, remained on the ceasefire it called in the aftermath of the Omagh bombing - the largest single atrocity of the Troubles. No one is much convinced.
The Continuity IRA was predominantly seen as a few grey-haired ideologues linked to Republican Sinn Fein. Led by the former IRA chief Ruairi O Bradaigh, it broke away in 1986 over Sinn Fein's decision to recognise the Irish parliament - a key step in the Adams-McGuinness strategy.
The Continuity IRA, which has never killed anyone in Ireland, is the only republican outfit not on ceasefire. Its name has been used to claim attacks this year which were the work of the Real IRA, to the fury of its more philosophically pure figures.
There was a short warning when dissident republicans attacked a hotel in Co Fermanagh in February, just as the tumultuous developments leading to the suspension of the devolved institutions were getting under way. The blast was claimed by Continuity IRA, though one of its leading figures later denied it.
Dissidents gave no warnings in March when they attempted to bomb an army barracks at Ballykelly, Co Londonderry and Dungannon, Co Tyrone. Nor was there any claim of responsibility.
They also tried to bomb Ebrington barracks in Londonderry, an army base in Co Fermanagh and the Glassdrummond army watchtower in south Armagh. The RUC in March intercepted a 500lb homemade car bomb outside Hillsborough, Co Down. It is believed it was intended for Thiepval Barracks in nearby Lisburn, which was bombed by the IRA four years ago.
Until yesterday dissidents had failed to mount an attack in Britain. Three youngsters were jailed last year after they were caught in an amateurish effort to firebomb stores in London. A car bomb was intercepted in April 1998, the month the agreement was signed, as its driver was taking it from Dublin to Holyhead.
Semtex was seized in Dublin last week and in an earlier raid on a training camp, so the dissidents have now breached IRA security. Security sources fear a unit is in place in south-east England.
While they remain confident the Real IRA has neither enough expertise or widespread support to mount a concerted campaign, either in Northern Ireland or Britain, they believe it could carry out a one-off spectacular. And they know that might be all they need to bring the fragile peace process crashing down.
