The arrests made yesterday will come as a relief to all in Britain and are a tribute to a police force that has passed one of the most severe tests in its history. All the perpetrators of the failed attacks of July 21 have been apprehended, yet the notion that Britain has suddenly been rendered entirely “safe” or “secure” is illusory.

Unfortunately, no matter how positive the short-term news, and no matter how efficient our police force, the vigilance must remain. The character of life will have to change, not necessarily dramatically but sufficiently to minimise risks and maximise the chance of terrorists being caught before they are able to slaughter innocent people. The abnormal will become normal. Politicians, the judiciary, the intelligence services, the police and the public will have to look at Islamist terrorism not as a theoretical concern whose extent might be debatable, but as a practical reality that can no longer be denied. To do so is not to afford the fanatics a form of victory but to be realistic about the facts.

This change can already be seen in debates that would have been unimaginable a month ago but are now being conducted with vigour. Should suspected terrorists be dealt with by guns or high-tech alternatives such as the Taser stun weapon? Yesterday’s arrests were apparently made without a shot being fired, but when is the “shoot-to-kill” order appropriate? What can be done to reduce the chance of another funeral of an innocent man such as that held yesterday in Brazil? If the London Underground is to have the kind of massive police presence that has been deployed over the past few weeks, will thousands more officers have to be recruited to compensate for these officers elsewhere? The rise of terrorism does not mean the marginalisation of more orthodox categories of crime.

Life does, however, change all the time even if some change is more agreeable than others. There was a point in the mid-1970s, when the Marxist-Leninist branch of the IRA appeared to have completely triumphed over the traditional nationalist wing, during which Irish terrorism took on an especially sadistic quality. It did not last long, but it was exceptionally bloody. Citizens throughout Britain were vulnerable to random bombings. Collective behaviour had to alter then, as it must in the face of an Islamist psychopathy that combines a desire to kill, which is at least as strong as that of the IRA at its most intense, with technology that advances the business of mass murder and heightens digital delusions of otherworldly paradise.

There is likely to be a pattern in this contest. Britain will adjust and become more careful, and this shift will hinder the terrorists. The old, “soft” targets will become harder for them to attack. After that, unfortunately, they too will think afresh and come up with new ways to create terror. Society will then have to reassess its position again.

This macabre arrangement will not continue indefinitely. The numbers of young men willing to be brainwashed into a state where they will take their own lives is not, despite the bravado from al-Qaeda spokesmen, infinite. The organisations that produce the equipment of death and destruction are capable of infiltration. Democracies have a quiet strength that sees them through difficult times. Living with terrorism is today a necessity. It is also a challenge that we have met and must continue to meet.