The BBC this morning reports a rather interesting story from Germany: the government has approved a plan to create a database of far-right extremists, as a response to the discovery of a neo-Nazi cell believed to be responsible for a string of murders.

“The register is to include the names of extremists who support neo-Nazism verbally and back or plan extremist violence. It will be modelled on a US database of extremist Islamists set up after the September 11 attacks.”

Presumably the idea is that its use would be confined to police and intelligence agencies, rather than be publicly available in the fashion of some s-x offender registers. Even so, the plan raises some obvious civil liberties questions, and difficult issues about the boundaries of legitimate political activity.

Granted, Germany has (understandably enough) a fairly low regard for the civil liberties of neo-Nazis, as shown by its laws on Holocaust denial. Plans are already afoot for another attempt to ban the far-right political party, the National Democratic Party (it survived a previous attempt in 2003) for links to neo-Nazi violence, and most Germans would probably say that national co-ordination of intelligence in the area is long overdue.

But the problem is a much more general one. We know from long experience that surveillance of political dissent can easily get out of hand; Martin Ferguson’s call for increased monitoring of environmental groups is only the latest example of insecure figures in authority trying, in effect, to criminalise peaceful protest.

Yet to say that people should be immune from government intelligence-gathering as long as their activities are lawful is untenable — firstly because it’s impossible to know whether they’re lawful of not without some sort of investigation, and secondly because even if it were it would limit us to response rather than prevention.

Police and intelligence agencies will always have to make judgements about what directions violence might come from, and to keep tabs in some way on people and organisations that seem liable to cross the line into criminal actions. The best we can do is to try to build as many safeguards as we can into the system to prevent abuse and to stop surveillance being used for political purposes.

That means that authorities need to resist the temptation of grand conspiracy theories that say, for example, that every Muslim (or every environmentalist) is a potential terrorist. But they also need to be on guard against the opposite mistake, of treating every crime in isolation and failing to notice important connections between them – connections that, if followed up, could help prevent further crimes.

And here we get into the west’s strange double standard about terrorism. As I’ve pointed out before, political-religious motives that count as evidence of terrorism when Muslims are in question somehow seem to count the other way when it comes to white-skinned right-wing terrorists like Anders Breivik.

My friend Chris Berg argued last month, quite correctly, that it’s wrong to blame the right as a whole for Breivik’s actions. But in doing so I think he missed the point. The sort of wild, paranoid rhetoric that may encourage violence has been tolerated by the mainstream right in a way that has no real parallel on the left. Nobody regurgitates chunks of David Marr or Phillip Adams in their manifesto to justify a massacre; the correspondingly violent polemicists of the left are way out on the margins, but Mark Steyn and Daniel Pipes have been brought within the tent.

A new assessment of Breivik’s sanity has now been ordered (note again the curious absence of the word “terrorism” from the report), so we don’t yet have the final word on his motivations. But we know the sources he was drawing on, and without pretending to offer any sort of medical diagnosis, his actions appear to be “rational” in a sense that, for example, those of the lone gunman who fired shots at the White House last November fairly obviously were not.

Those who agree with Breivik’s politics should be at liberty to say so and to advocate their views peacefully. But those who want to be counted as part of the “mainstream” shouldn’t be giving them comfort.