For a nation that has remained relatively insulated from the terrorist violence that roiled the continent in recent years, the sight of British troops patrolling mainland UK streets — one consequence of the terror threat level rising to “critical” — will come as a particular shock. It will raise the spectre of 2003, when Tony Blair sent 400 soldiers with armoured vehicles to Heathrow, and the dangerous years that followed, when security services feared that they were losing their grasp on the problem. The bombing of Manchester Arena on Monday night is redolent of that period. It is the worst terrorist attack on Britain since the July 7 London bombings more than a decade ago and the worst-ever attack on Manchester and the north. But how new and advanced was this atrocity?

Former officials of the National Counter Terrorism Office (NCTO) have suggested that the attack was “sophisticated”. This is true, insofar as it represents a step up from the low-technology vehicle and knife attacks seen in the UK over the past 12 years. Building a bomb is significantly harder than procuring a van or a blade, and entails several steps — research, acquisition of materials, and perhaps collaboration — that increase the probability of detection by the intelligence services.

Yesterday’s decision by MI5’s Joint Terrorism Analysis Centre (JTAC) to raise the threat level for international terrorism, meaning that an attack is expected imminently, probably reflects home secretary Amber Rudd’s acknowledgment that 22-year-old Salman Abedi “likely?.?.?.?wasn’t doing this on his own”, that such a bomb would have required assistance, and that one or more accomplices therefore remain on the loose.