Nothing and nobody can stop bombs going off. No citizen, no police force, no army, no government and no global military alliance can prevent a determined suicide bomber from blowing himself up. It will happen and innocent people will die as a result, horribly, as they do on the roads, from drugs and alcohol, or from natural disasters - again without responsible authority being able to stop it.

What is recent is the admission of this truism into the mainstream of government under the rubric of "terrorism". This week two outgoing presidents, America's George Bush and Pakistan's Pervez Musharraf, defined their terms of office in relation to terror. Bush did so in his final state of the union message on Monday and Musharraf that same day in London during a charm offensive prior to next month's elections.

To Bush, the "war on terror" is the ruling mantra of his politics of fear. Since 9/11 gave a prop to his weakening presidency, his language has scaled new heights of alarmist rhetoric. It has validated every internal repression and every external war. "He who is not with us is against us," he cries. Terrorists everywhere are "opposing the advance of liberty ... evil men who despise freedom, despise America and aim to subject millions to their violent rule".

As the sociologist Ulrich Beck has written, "properly exploited, a novel risk is always an elixir to an ailing leader". By declaring a threat so awful as to be intolerable, a politician can limit the liberties of a free society in the name of risk-aversion. Musharraf utters hardly a sentence that does not contain the word terror. Pivotally close to the base from which 9/11 was apparently launched, his dictatorship has been indulged by London and Washington for a full seven years. This week Gordon Brown hailed him as a "key ally on terrorism", enabling him to take comfort in sacking his judiciary and curbing his media.

Had the war on terror been used only as a metaphor for better policing, like rhetorical "wars" on drugs, poverty and street crime, it might have passed muster. Bush and Musharraf have found the military metaphor too potent to resist and duly carried it into literal effect. The result has been a disaster for their countries, and incidentally for themselves.

The west's Afghan adventure is now devoid of coherent strategy. Soldiers are dying, the opium trade is booming and aid lies undistributed. Command and control of the war against the Taliban is slipping from the most bizarre western occupying force since the fourth Crusade to a tight cabal around the Afghan ruler, Hamid Karzai, who is fighting to retain a remnant of authority in his own capital.

Karzai's exasperation with the west has led him to refuse the services as "coordinator" of the former Liberal Democrat leader, Paddy Ashdown. The latter may have cut a dash in the subsidy swamp of Sarajevo, but in Afghanistan he would have been a boy on a man's errand. Karzai knows well that his fate lies not with the patronising platitudes of western proconsuls but in the hard graft of provincial warlords, drug gangsters and Taliban go-betweens.

These go-betweens have had their status massively boosted by the war on terror. Bush's demand in 2001 that Musharraf "join the war" sent Pakistani forces into the border territories, breaking old treaties and driving the Pashtun tribes into the eager arms of Taliban leaders. This undoubtedly saved Osama bin Laden's skin from the fury of the northern Tajiks, committed to avenge his murder of their leader, Ahmed Shah Massoud.

Musharraf, at America's bidding and with $10bn of American money, has done what even his craziest predecessors avoided, and recklessly set the Pashtun on the warpath - increasingly in thrall to a revived al-Qaida. The result is a plague of suicide bombings and killings in the heartland of his benighted state. From the law courts of America to the mosques of west London and the mountains of the Hindu Kush, the war on terror has been lethally and predictably counter-productive. It embodies the new stupidity in international affairs.

Nobody disputes that there are killer cells at large in the world, most of them proclaiming various Islamist creeds. It is the job of intelligence agencies and the police to catch as many as they can. After a hesitant start, they appear to be quite good at it. Some bombs will get through but they will not be deterred by draconian laws, any more than by machine gun-toting policemen in Downing Street and Heathrow. Robust societies can handle this admittedly intermittent threat. Only weak ones will capitulate to it.

The menace of these killers lies not in their firepower but in their capacity to distort the judgment and commitment to freedom of politicians too cowardly to bear on their shoulders the burden of risk. In two weeks' time, the fragile democracy of Pakistan will defy the bombers and hold an election prior, it is hoped, to some version of democratic rule. Such communities will defy a probable burst of terror bombs only if their leaders stop setting "terrorists" on a pedestal and using language that exaggerates their capacity, as Bush puts it, "to oppose the advance of freedom".

It is leaders, not bombers, who have the power to balk the advance of freedom. Already those leaders have used the war on terror to introduce the Patriot Act, Guantánamo Bay and a $1.5 trillion war in Iraq. In Pakistan they have used it as an excuse for emergency rule, the imprisonment of senior judges, and the provocation of unprecedented insurgency in the north-west frontier territories. In Britain leaders have used the war as an excuse for 42-day detention without trial, the world's most intrusive surveillance state, and not one but two contested military occupations of foreign soil.

This so-called war on terror has filled the pockets of those profiting from it. It has killed thousands, immiserated millions and infringed the liberty of hundreds of millions. The only rough justice it has delivered is to ruin the careers of those who propagated it. Tony Blair was driven to early resignation. Bush has been humiliated and Musharraf's wretched rule brought close to an overdue end. It may be an ill wind that blows no good, but it is hardly enough.

simon.jenkins@theguardian.com