What public purpose is served by the prime minister declaring she has raised Britain’s “threat level” to “critical”? Before she thought another terrorist attack was “highly likely”. It is now “expected immediately”.

What are we supposed to do with this information, other than feel vaguely alarmed? The words can have meaning only in the wartime sense, of ordering us to put on gas masks and head for bunkers. Do we alter our journeys to work? Do we put on body armour? Do we keep away from crowded places? As for sending in the army – terrorism’s propaganda coup – what good does that do? It supposedly releases the police, but to do what?

For the past 48 hours we have witnessed the impressive calmness of the people of Manchester. They have not shown anger or clamoured for revenge, as in more hot-headed societies, just an intense grief over young lives pointlessly lost. Their prayers, candles, poetry and song have been those of a community closing round its bereaved with dignity. The accidental death of anyone for any reason is private agony enough, but when so many are involved, it is right for public figures to express a more distant sympathy.

Sympathy is quite different from aggressive language and bombastic response. We can agree with Theresa May that the Manchester attack was sickening, callous, warped, twisted, even cowardly, and the other adjectives with which she deluged it on Tuesday. We might wonder if she stopped for a moment to ponder the wisdom of turning personal tragedy into such purposeful aggression, knowing it would be greeted with glee by terrorism’s supporters. A similar rush to rhetoric on May’s part led to a severe over-reaction to the Westminster deaths in March.

Open societies have no defence against suicide bombers. Counter-terrorism manuals assert this, over and again. The conduct of public security – such as swamping places with police, closing streets and searching bags – merely redirects the determined bomber elsewhere. Terror bombing is the one foolproof weapon of the weak against the strong. We cannot screen every public space or search every pedestrian. There is nothing new to this. The car bomb and the terror grenade are as old as Conrad’s secret agent, and his “pestilence” which stalks the street with death in its pocket.

All we can hope to do is enter into the minds of the bombers and their associates to prevent them at source. That is essentially a covert activity, and is clearly in its infancy. We can try to clean the pool in which fanaticism swims, the ideological grooming and conditioning. The security services must relentlessly infiltrate Islamist networks. That is their job – they claim to foil a dozen attacks a year – but publicising it cannot be necessary.

When accused of seeking political capital from terrorist outrages, politicians explain that their job is to “express public outrage and show that something is being done”. That is cynical. Mancunians this week expressed grief, not outrage. A leader’s job is to allay rather than promote public anxiety. It is to minimise rather than propagate the impact of an incident.

The key text here remains the reaction of New York’s mayor Rudy Giuliani on the afternoon of 9/11, when he called on New Yorkers to stay calm, “buy a pizza, take the kids to the park, see a show”. May makes no such statements. She does not even do what most prime ministers were careful to do during the IRA terrorism, which was to treat it as criminal, however heinous, and leave it the home secretary to respond.

May seized the megaphone for herself, and thus inevitably politicised the event. There was murmured talk of states of emergency and of suspended election campaigns. Armed troops were deployed in British cities. Parliament is shut down. Buckingham Palace’s guard is not changed. This is a statist response on a par with Tony Blair’s deployment of Scimitar tanks to Heathrow in 2003 as a “deterrent to terrorism.” The sole consequence was an instant, multimillion-pound hit to the tourist industry.

The prime minister yesterday plunged twice into Cobra, ignoring Thatcher’s maxim of “Deny terror the oxygen of publicity”. She was like David Cameron during last year’s Brussels massacre, when he put 10,000 troops “on standby”, assigned 22 helicopters to the capital and donated £143m to further counter-terrorism measures. He thus encouraged Donald Trump to his absurd conclusion that Belgium and France, if not the whole of Europe, “are literally disintegrating”.

The politics of fear is the oldest sick trick in the book. The idea that tanks, troops and helicopters can deter a suicide bomber in a crowded space is ridiculous. The only people it deters are foreign tourists. It shows the madness that seizes all ministers who descend the murky depths of Cobra. They think they are suddenly Churchill’s children.

If another attack occurs, as it always may, who can tell where or when? The perpetrator will have just one aim, to gain publicity for a cause. Bruce Hoffman, in his classic book Inside Terrorism, constantly emphasises the role of “psychological repercussions beyond the immediate victim or target.” Terrorism’s objective is not just to kill but “to create power where there is none, through the publicity generated by the violence”. Publicity is terror’s “second wave”. Without publicity, terrorism is just dead bodies.

Today’s terrorist wants to frighten the enemies of Islam into curbing liberties and oppressing Muslims. That is why a genuinely “tough” response to Manchester would be opposite of May’s. It would be to let the city bury its dead in peace, not surrounded by her guns. It would plead with Muslim leaders to look to their own. It would beg the media not to play terror’s game, by relentlessly publicising its violence and harassing the victims’ families.

Above all, a tough response would point out that terrorism is aimed at our freedom to congregate. It is the price of that freedom. For all its horror, the risk of death from terrorist violence is minimal. True toughness would downplay it, avoid the grandstanding and empty rhetoric, the machismo of soldiers and gunships. It would avoid the easy slither into a bruised and weakened liberty that is now the most menacing threat.