It was several years into the campaign before the authorities accepted that they faced not just another series of hit-and-run attacks but a calculated attempt to bring to Britain many of the features of the Northern Ireland campaign.
Belatedly, the authorities have attuned themselves to countering a campaign which shows all the signs of running for many years. One indication is the existence of a prolonged research programme into matters such as the electronic searching of large vehicles. Another sign was the decision to give MI5 the lead in dealing with gathering intelligence on the IRA in Britain, rather than the Special Branch. Only operational since October, this transition is still taking place, some sources believing it will take many more months before MI5 is firmly in the saddle.
Reliable sources acknowledge that the Special Branch was at first extraordinarily reluctant to envisage any such change, but say this week's reports of continuing tensions between the organisations are exaggerated.
No single individual or agency is in charge of the response to terrorism. Those involved include MI5, the Special Branch, the Anti-Terrorist Branch at Scotland Yard and the 42 provincial forces, each with a chief constable and their own detectives, whose sensibilities have to be accommodated.
Above their heads is a network of Whitehall committees which supervise the wider intelligence community of which counter-IRA terrorism is now a significant part.
Commander David Tucker, the head of the Anti-Terrorist Branch, is deemed the national co-ordinator and theoretically controls all post-incident investigations.
A committee of the Association of Chief Police Officers, which consists of Brian Johnson, the Chief Constable of Lancashire, Albert Pacey, the Chief Constable of Gloucestershire and chairman of Acpo's crime committee, together with Mr Tucker's superior, Bill Taylor, the Assistant Commissioner (Specialist Operations) at Scotland Yard, exists to iron out differences.
The main problem may lie not in a lack of co-operation but in a basic lack of information on the terrorists. The sketchy nature of this was illustrated by the fact that security was stepped up a week before the City of London attack, with extra police activity and warnings to a number of VIPs to be particularly vigilant.
Security appears to have slackened, however, just before the bombing took place. Friday's routine alert of IRA activity is believed to have been too general to enable forces to take specific precautions.
Many aspects of the IRA campaign are still shrouded in mystery. The organisation appears to have a network of members and supporters in Britain, but these may not carry out operations. Senior police have now revised their figures upwards and believe that more than 30 hardcore activists may be in Britain at any one time, comprising those resident and those dispatched for particular missions.
The intelligence services do not, however, appear to know how those activists travel back and forth to Ireland. They have lists of people they suspect of being involved, but much of this information is speculative and incomplete. One source said: 'The picture is fuzzy on how and by what routes this material is moved, and about some of the people who are being used for operations in Britain, who are clearly not known to police in Belfast, Dublin or London.'
The RUC and Garda in the Irish Republic report suspicious disappearances of known 'players', but there seems to be no reliable early-warning system to alert London.
Questions about the effectiveness of intelligence-gathering have led to increased pressure on the Prime Minister to introduce legislation to allow oversight of MI5 by MPs. Senior Tories have been assured privately that an MI5 Bill will be included in the legislative list for the Queen's Speech in November.
Senior backbenchers were last night seeking renewed assurances that there would be no slippage with the Bill in the wake of the City bombing. There is concern at Westminster that MI5 has failed to penetrate the IRA cells in England.
The City of London police are not being blamed by the MPs for the bombing. They have accepted assurances from the police that there was nothing that could be done to prevent the bomb going off.