The police and MI5 appeared last night to have thwarted one of the 30 top priority terror plots currently under investigation. The arrests in Birmingham were the culmination of an operation that began with a tip-off six months ago.

Dozens of counterterrorist officers kept suspects under round-the-clock surveillance, bugging their homes, cars, and telephones and noting whom they met and where they went. This operation was under way when the head of MI5, Eliza Manningham-Buller, issued her sober warning last November about the nature of the threat.

She said British-based Islamic militants linked to Al Qaeda would use chemical, biological, or nuclear weapons if they got the chance. Dame Eliza said MI5 was monitoring some 200 networks, with more than 1,600 individuals “actively engaged in plotting or facilitating, terrorist acts here and overseas.”

The security service has now expanded its operations and has regional bases from which to carry out these inquiries. West Midlands police have also set up their own counterterror unit and no longer rely on Scotland Yard. This is its first major investigation.

Most of the plots being monitored involve mass casualties and indiscriminate carnage. The Birmingham conspiracy was fundamentally different, involving the “Iraqi-style” kidnap and beheading of an Islamic soldier. Police said it was “unlike anything seen before in the U.K.”

But terrorists in Northern Ireland used these tactics for 30 years. During that period, abducting, torturing, and murdering people from within the local community for collaborating with the “enemy” were regular occurrences.

And targeting a soldier was not a first for British-based Islamic terrorists. Last year, Abu Baker Mansha, 21, was jailed for six years for plotting to kill or harm Corporal Mark Byles, who had won a Military Cross in Iraq.

Targeting an Islamic soldier was reminiscent of the IRA’s determination to control “their” communities. For the Republicans, there was no greater criminal than the “traitor” who consorted with the enemy. No one took a higher risk than the Roman Catholic who joined the RUC or the local lad who signed up for the army.

Soldiers from the province could not travel home in uniform and would always have to watch their backs. Civilians who helped the security forces were kidnapped, tortured, and left for dead with a hood over their heads.

The alleged Birmingham plot was grotesque because it involved filming the atrocity and putting it on the Internet. But the IRA or Loyalist killers would have done the same had the technology existed. They often released photographs of their victims.