Tony Blair was talking to soldiers he had sent to fight the toughest guerrillas on earth for control of southern Afghanistan. He told them: "Your defeat [of the Taliban] is not just on behalf of the people of Afghanistan but the people of Britain ... We have got to stay for as long as it takes."

The prime minister's brain has clearly lost touch with reality. Even under the Raj there was no conceivable way Britain could conquer and hold the arc of territory to which Blair was referring. It stretches from the Persian Gulf through Iranian Baluchistan and Afghanistan to Pakistan. No central government has come near to controlling this region, and its aversion to outside intervention is ageless and ruthless, currently fuelled by the world's voracious appetite for oil and opium. But it poses no threat to world security.

The sole basis for Blair's statement is Mullah Omar's hospitality to the fanatic, Osama bin Laden, at the end of the 1990s. As we now know, this was never popular (an Arab among Pashtuns); after 9/11, when the Taliban had collaborated with the west over opium, either Bin Laden would eventually have had to leave or the Tajiks would have taken revenge for his killing of their leader, Sheikh Massoud. Even the Pakistanis were on his tail. Either way, Talib Afghanistan was no more a "threat" after 9/11 than were the American flying schools at which the 9/11 perpetrators trained.

So what is Blair getting at? He once confessed to his hero, Roy Jenkins, that he regretted not having studied history at Oxford. He never spoke a truer word. The concept of world security as holistic and vulnerable to incidents such as 9/11 is nonsensical. Politics is not a variant of the Gaia thesis, in which each component of an ecosystem depends on and responds to every other. There is no butterfly effect in international relations. For want of a victory in Helmand, the Middle East is not lost, nor for want of victory in the Middle East is western civilisation lost.

This is as well, since Blair's resumed war in Afghanistan is clearly not being won. We know from the former army chief Lord Guthrie that Blair, despite promising to "give the army anything it takes", has refused the extra troops and armour needed by the pathetically small expeditionary force of 7,000 in Helmand. He has already had to switch tactics from winning hearts and minds to American-style "search and destroy", blowing up villages with 1,000lb bombs (as we saw on TV last week). British commanders are describing "successes" in terms of enemy kills. They should recall that Victorian officers in the Punjab were told that such boasts would be treated as a sign of failure, not success. Such killings infuriated the population and presaged revenge attacks. Has the British army learned nothing?

Blair has not been able to persuade his Nato allies in Europe of his apocalyptic world-view. The use of the word terrorism to imply some grand military offensive against the west may sound good in White House national security documents and Downing Street speeches. But terrorism is not an enemy or an ideology, let alone a country or an army. It is a weapon, like a gun or a bomb. It is not something that can be defeated, only guarded against.

Nor can terrorism ever win. Blair's flattering reference to it was in reality to al-Qaida and to the Islamist jihadism whose cause he has so incessantly advertised. As the American strategist Louise Richardson points out in What Terrorists Want, al-Qaida has not the remotest chance of defeating the west or undermining its civilisation. Only a deranged paranoid could think that. Some group or other will always look for ways to commit random killings, against which national security services need to be vigilant. But this is not war. Richardson points out that these groups are being grotesquely overrated. They cannot plausibly deploy weapons of true mass destruction, and remain stuck with the oldest terrorist tool of all, the man with a bomb (and if we are really negligent, with a plane).

While terrorism can take on different guises, it is not new and is not a threat to human society to rank with a world war or a nuclear holocaust - as the home secretary, John Reid, has absurdly claimed. Terrorist incidents are the outcome of someone's mental pathology and are of no political significance - unless cynical leaders in a targeted community choose otherwise.

What is sad about Blair's statement is not its strategic naivety but the psychology behind it. Why have the leaders of Britain and America felt driven to adopt so wildly distorted a concept of menace? In an analysis of terrorism in the latest New York Review of Books, Max Rodenbeck offers plausible but depressing answers. They include the short-term popularity that war offers democratic leaders, the yearning of defence chiefs and industries to prove the worth of expensive kit and, in Iraq's case, "the influence of neoconservatives and of the pro-Israeli lobby, seeing a chance to set a superpower on Israel's enemies".

All this is true, but I sense a deeper disconnect. The west is ruled by a generation of leaders with no experience of war or its threat. Blair and his team cannot recall the aftermath of the second world war, and in the cold war they rushed to join CND. They were distant from those real global horrors. Yet now in power they seem to crave an enemy of equivalent monstrosity. Modern government has a big hole in its ego, yearning to be filled by something called a "threat to security".

After 1990 many hoped that an age of stable peace might dawn. Rich nations might disarm and combine to help the poor, advancing the cause of global responsibility. Instead two of history's most internationalist states, America and Britain, have returned to the trough of conflict, chasing a chimera of "world terrorism", and at ludicrous expense. They have brought death and destruction to a part of the globe that posed no strategic threat. Now one of them, Tony Blair, stands in a patch of desert to claim that "world security in the 21st century" depends on which warlord controls it. Was anything so demented?

simon.jenkins@theguardian.com