The voice is unmistakable, the message as delusional and the timing as opportunistic as ever. Six days before George Bush leaves office, Osama bin Laden has broken his fugitive's silence to taunt and humiliate the nation to which he brought such evil. An audiotape posted on al-Qaeda websites yesterday called on the world's Muslims to wage a new jihad over Gaza. But its real message is addressed to America. Gloating over his continued survival, he makes a stark and ugly point: despite nearly eight years dedicated to the defeat of al-Qaeda, President Bush's War on Terror has been hamstrung by strategic and political failure.

At first, America focused its anger and military might on the willowy figure spewing out uncompromising messages of hatred. Killing or capturing the man who masterminded 9/11 was the absolute priority, which drove the decision to invade Afghanistan, overthrow the Taleban and pursue bin Laden and his acolytes into the mountains. Washington underestimated the challenge. A reward of $50 million is a vast sum. Yet no disgruntled zealot, no bounty-hunting tribesman has betrayed bin Laden's whereabouts or tipped off the US forces combing the Hindu Kush.

The Pentagon also seemed unprepared for the sheer difficulty of co-ordinating a manhunt in landscape so rugged and unfamiliar. Mistakes were made in liaison between units and forces. An attempted ambush in the Tora Bora mountains failed to net bin Laden or any of his accomplices who were apparently able to slip across the Pakistani border into the badlands of Waziristan. The US hoped to track telephone signals but was thwarted by al-Qaeda's reliance on couriers, tribal omertà and clandestine night movements.

Washington's second big failure was then to switch tactics. Bin Laden, spokesmen said, was increasingly irrelevant. Isolated, probably ill and unable to direct al-Qaeda's operations abroad, he was no longer an immediate danger to the West. Instead, US Intelligence concentrated on trying to dismantle terrorist networks across Europe and the Muslim world, on interrupting operations planned in Pakistan or Iraq and on capturing or killing men such as Abu Musab al-Zarqawi, the Jordanian mastermind of terrorist atrocities in Iraq.

Bin Laden, however, was not irrelevant. He and Ayman Zawahiri, his fanatical Egyptian deputy, continued to post videos on websites and Arabic satellite channels, mocking the US failure to silence them, goading followers into further attacks and making it clear that they were fully abreast of all the terrorist atrocities carried out by al-Qaeda and its affiliates since 2001.

Their messages had only marginal impact on the Middle East. Bin Laden came late to the Palestinian cause, in which he showed little interest beyond its potential to rally Muslims around the world against America. Hamas owes nothing to al-Qaeda, which may seek to replace it amid the present chaos in Gaza. But bin Laden's simple survival has proved hugely important. To millions of angry Muslims and others chafing at US power, he has become a symbol of resistance. Even those who found his beliefs repulsive were somehow heartened by his defiance. And this is a message well understood by voters in the West. The Pentagon may think him a spent force; for millions of Americans, he is the proof of their own inability to bring him to justice.

Too late, Washington has returned to a policy of decapitation with its intensive use of drones to kill al-Qaeda's leaders in Pakistan. This is proving extremely effective. But its real target remains untouched. Mr Bush leaves office, as his father did, with unfinished business abroad. It is a strategic failure and a brutal legacy for his successor.