They shot Constable Jim Doherty just after 8.30am on November 8 2007, as he sat in his car in slow-moving traffic in Londonderry, after dropping off his son at his Catholic school in the city.

He represented a new type of policeman, a working-class Catholic who joined the reformed police force which has come into being as part of the new political dispensation.

A close friend said of Jim Doherty that in joining the police he did what he believed was best for his family and for society. “He obviously thought that times have changed and things like this would never happen,” the friend said. “Unfortunately it happened to a really good person.”

Jim Doherty was struck in the arm and face by shotgun pellets. It was his ill-fortune to be targeted by one of the small bands of republican gunmen who are still active. But then came a stroke of luck: a second man appeared with a handgun to finish him off, but the weapon jammed. The policeman survived and is now recovering after surgery, much to the disappointment of those who attacked him. They said in a statement, part menacing, part truculent: “He might not be so lucky next time.”

It was not an isolated incident but one of many, and there are few indications that the security forces are close to eliminating these republican renegades. The police assessment is, in fact, that the dissident threat is at an all-time high.

According to the nationalist commentator Dr Brian Feeney: “There's no doubt they are trying to provoke a reaction. Ideally they want to kill a policeman in order to get the army back on the streets. They want the nationalist population to get oppressed again.” The leader of Sinn Fein, Martin McGuinness, scornfully echoed this point: “Do they really want to see twenty or thirty thousand soldiers back on the streets? Those people think that more car-bombing, more military activity, is going to bring about the freedom of Ireland. They're living in cloud cuckoo land.”

These groups, a modern version of what in the 1920s was referred to as the legion of the rearguard, disapprove of the peace because it is bringing the wrong sort of peace. This was spelt out bluntly by a leading Derry dissident, Gary ‘Donzo’ Donnelly. He declared defiantly: “I believe whilst there is a British presence in Ireland there will be people who will resent it.”

The groups who share such sentiments are not numerically strong but they remain dangerous. In fact, Sir Hugh Orde, the Chief Constable, warns that they vie with each other. “What worries us,” he said, “is that there is probably an appalling competition between the dissident republican groups to be successful. In their eyes this would mean to kill a police officer. They are determined to kill an officer — they are just determined to cause mayhem.”

The renegades mainly in fact kill their own members. The half-dozen deaths they have been responsible for in recent times include a couple of uninvolved civilians, but mostly they include members of their own groups, killed for internal reasons.

There are at least four active groups. The oldest, the grandiosely titled Irish National Liberation Army (INLA), has been in existence since the 1970s and has carried out three killings in the last few years. According to an official intelligence assessment of the INLA: “We believe that it retains a desire to be able to mount attacks. We continue to believe it is a threat and has the capacity for extreme violence.”

Another newer grouping calling itself ONH emerged last year, quickly becoming involved in targeting, recruiting and training new members as well as attempting to raise funds and obtain weapons. Its name is an abbreviation of Oglaigh na hEireann, which translates as ‘the army of Ireland’. ONH's internal discipline might be described as stern: it has already exiled one disobedient member and killed another in an internal dispute. The security forces regard it as a ‘continuing and serious threat’.

The Continuity IRA has been in existence for many years and in recent times has been responsible for two internal killings as well as attacks on police officers, vehicles and premises. It is described as ‘active, dangerous and committed, and capable of a greater level of violent and other crime’.

But of the violent groups, the Real IRA is the best-known, or rather the most notorious. It was they who shot Jim Doherty, they who caused the damage to a major shopping centre. Above all, they killed 29 people in the 1998 Omagh bombing. The subsequent incarceration of the group’s founder Michael McKevitt, now serving 20 years in the Republic for directing terrorism, did not halt Real IRA violence. Two years after Omagh they mounted a ‘spectacular’, with a rocket attack on the London headquarters of MI6. Although this caused little actual damage it was an undeniable coup for the dissidents, announcing they were back in business.

MI5 and the Special Branches in both parts of Ireland pour huge effort and huge resources into combatting the dissident republicans, with some success. Efforts to smuggle in weaponry through Lithuania and the Balkans have been thwarted. An unusually high percentage of dissident operations go wrong, with weapons and explosives regularly seized and arrests made. Their members often appear in court, and a total of more than 90 dissidents are currently in prison in the two parts of Ireland.

The universal belief is that many of the little groups have been penetrated by the security agencies, often using sophisticated surveillance. And there have been unexplained oddities during legal proceedings, with certain defendants being spirited away from their former colleagues. One Belfast man convicted of involvement in dissident activity was taken into protective custody, appearing in court protected by eight prison officers. He has now vanished from the prison system and is assumed to be somewhere in Britain in hiding from colleagues who regard him as a security force agent and would dearly love to kill him. Everyone assumes he is not the only dissident recruited by the intelligence agencies.

There is undoubtedly a permanent secret battle going on between the dissidents and the intelligence services. The latter can claim credit for preventing any attacks in Britain since the MI6 rocket incident, and for putting scores of dissidents behind bars.

But the attack on Constable Jim Doherty indicates that, despite all the intelligence effort, there are still parts of the dissident underworld which remain unpenetrated. This much was frankly admitted by the Chief Constable when he explained: “They are well infiltrated both north and south of the border, and are paranoid about their activity because we keep disrupting it. We keep arresting people, we have good coverage and these groups are badly disrupted. But the harsh reality is we have never had a full intelligence picture. Any successful crime in a way is an intelligence failure.”

So more than a decade of effort has not eradicated dissident activity; in fact it is clear that enough new recruits are joining up to keep the groups going. In the early days, seasoned ex-IRA operators formed the backbone of these organisations. But today, according to Sir Hugh: “The people we are arresting are not 50 or 60-year-olds from the old world. These are young people who are being targeted by dissidents — disenfranchised, marginalised young people who they are now using to do their dirty work.”

The new political system at Stormont is taking time to bed in, with a continuing trial of strength between Sinn Fein and loyalist politicians. The Assembly is meeting, but the ruling Executive is not. Sinn Fein has to thrash out a deal with the loyalists, but it also has to confound the dissidents by demonstrating that politics can deliver. The hard core of dissidents is never going to be convinced the gun should be taken out of Irish politics, but the hope is that an effectively functioning system of powersharing in Belfast will eventually undermine them.

This intertwining of high politics and low terrorism is something that concerns the authorities, who want to get the political show on the road.

They already detect a new attitude towards the police, a senior officer reporting: “There is a huge wind of change out there. I have seen that in a number of investigations — there is a completely different response to police on the ground.”

A high degree of impatience was evident in comments from the Chief Constable: “The politicians need to nail the political vacuum in which these people are now operating. Until they do that, quite frankly I think the opportunities for recruiting more marginalised young people into this crazy activity will continue.”