We must get to the bottom of how disaffection can turn into violence

The Manchester attack would have added May 22 to the mournful litany of dates memorialising terrorist atrocities in the United Kingdom had it not already qualified as the day that Fusilier Lee Rigby met his death in Woolwich in 2013. These attacks highlight our vulnerability to the indiscriminate acts of people who see no better way to express their disaffection than by murdering their fellow citizens.

But it is hard to explain a level of disaffection that drives people to such random slaughter as happened in Manchester. Fusilier Rigby’s killers claimed to be avenging British Army action against Muslims and, however unjustified, he was at least a soldier. But the very similar claim made by Isil, which said that a “soldier of the Caliphate” had carried out the Manchester attack against “crusaders” to avenge their acts in “Muslim lands”, is a ridiculous description of the killing of children at a pop concert.

The motivation of the killer will become clearer as further details emerge. But although there may have been indicators of a predisposition to violence, it is likely that he fitted a profile common to thousands of others. In almost all cases, people who have committed acts of terrorism in Europe over the past few years have turned out to have had some sort of security or criminal record.

But the authorities cannot be expected to deal with the sheer volume of people of concern without resorting to an unacceptably intrusive investigation and a vast increase in staff. That would suggest a society that none of us would choose to live in. The authorities must therefore prioritise, and with many hundreds who have expressed support for terrorism, topped by more than a thousand who have returned from Syria or attempted and failed to get there, mistakes will be made. The inevitability that terrorists will succeed in carrying out an attack is why the threat level has remained at “severe” since mid-2014. It is unlikely to go down any time soon.

Clearly revolted by the Manchester attack, Andrew Parker, head of MI5, assured the public yesterday that his service continues to work with relentless focus on numerous counterterrorism operations. We would hope so. But why are there so many?

The other reassurance that he should offer is that MI5 and the Government are developing a better understanding of why people resort to such behaviour. There are millions who object to Western foreign policies but will never become terrorists. We need to understand what drives some into violence.

This requires a close examination of why people join – and leave – terrorist groups. That detailed knowledge can help Western societies understand why they have this problem and devise policies accordingly. We face the difficulty of dealing with returnees from Iraq and Syria and assessing the threat they pose. If we don’t know why they went and why they came back, this will be difficult.

Preventing future attacks will not get easier. What we need are policies that lead to a more cohesive society that refuses to be divided into “them” and “us”, whatever the provocation.