The new enemy is radical Islam, driven by both ideology and nationalism, and embodied by al-Qa'ida, its fury directed against the West in general and the US in particular. Already the "war on terror" has led America to embrace a new doctrine of preventive war. In its name, the US has invaded two countries - Afghanistan and Iraq - and some urge it to attack others. This amorphous conflict has already lasted longer than the Korean War and US involvement in the Second World War, and this may be only the beginning.
For George Bush and his promises, 2005 has not been a vintage year. But in the "war on terror", in one respect at least, he has been as good as his word. "Our response involves far more than instant retaliation and isolated strikes," he told Congress nine days after the attacks. "Americans should not expect one battle, but a lengthy campaign, unlike any other we have ever seen."
And so it has been - and much has been accomplished. The al-Qa'ida of 11 September 2001, it may be argued, no longer exists. Its leadership has been decimated. In Afghanistan, the Taliban regime that long gave Osama bin Laden shelter has been swept away. Intermittently, Bin Laden and his deputy, Ayman al-Zawahiri, issue videos and audio tapes, sometimes taunting, sometimes menacing, but whose main practical purpose is to remind the world they are still at large.
The pair are assumed to be penned up in the tribal lands and mountain fastnesses of the Afghan-Pakistan border, their communications uncertain at best, their control over their followers a matter of increasing debate. Regularly, reports surface that Bin Laden is dead or dying, and that Zawahiri is the senior operational figure of the old guard.
In their place, a new terrorist reality is emerging, of a network of groups perhaps less sophisticated than al-Qa'ida in its prime, but no less difficult to counter. It is inspired by Bin Laden but probably no longer run by him. Even between al-Qa'ida and Abu Musab al-Zarqawi in Iraq, its best known field commander and a former Bin Laden protoge, the relationship is unclear. Some counter-terrorism experts now see Zarqawi as the most important figure in the movement.
Confirmation of a kind came last summer with a letter, purporting to be from Zawahiri to Zarqawi, that fell into US hands in Iraq. In it, the old commander complains that Zarqawi's brutal methods, the videotaped beheadings of hostages and the rest, alienate ordinary Muslims and reduce radical Islam's appeal.
In some respects, military success against the terrorists has made the job of fighting them harder. Now there are new adversaries, less well known to intelligence services, with fluid command structures and ad hoc alliances that are hard to expose. The old al-Qa'ida and its allies have the ability to strike targets almost anywhere in the world: a nightclub in Bali, commuter trains in Madrid, Underground trains and a bus in London, banks and consulates in Istanbul, foreigners' compounds in Saudi Arabia, and a wedding party at a hotel in Amman, Jordan.
Yet there is one striking omission from the list. Since 9/11, despite many scares, there has been no terrorist attack on US soil, or even solid evidence of sleeper cells, similar to those that carried out the 2001 attacks. Hundreds of suspects have been rounded up, but only one person has been charged in connection with 9/11, and even now the precise role of Zacarias Moussaoui in the plot is far from clear.
For the rest it has been one false alarm after another. In mid-2002 the US government announced with much fanfare that it had caught José Padilla, an American citizen who converted to Islam, as he was plotting a radioactive "dirty bomb" attack. It held him incommunicado in a naval prison for three-and-a-half years. That charge has been quietly dropped. Padilla will now go on trial in civil court this month as outrider in a terrorist group active in Afghanistan, not in the US.
In December, Sami al-Arian, a Palestinian-American professor from Florida, was cleared of charges that he was a leader of the Islamic Jihad terrorist group which has carried out bombings against Israel - a case under investigation since the mid-1990s. Meanwhile, the June 2003 convictions of three Detroit men accused of being part of a "sleeper operational combat cell" have been overturned. The case had been the only successful post-9/11 prosecution of terrorists. But a federal judge ruled that some evidence against them had been fabricated, while exculpatory evidence had been withheld.
These episodes have not been a glowing advertisement for US justice. However, the astonishing fact remains that for more than four years terrorists have not struck on US soil. After all, what could be simpler than a suicide bomber blowing himself up at a shopping mall in the unprotected American heartland? "Suiciders", as Mr Bush calls them, cost little and are virtually impossible to thwart. Nor can it have escaped the attention of America's enemies that in the world's most avid consumer society, and one far less inured than Europe to terrorism, such an attack would have consequences - psychological and economic - far exceeding its intrinsic importance. But nothing has happened.
This absence is even more striking given the strictures of the bipartisan commission set up to investigate the 9/11 attacks over how little has been done to head off future terrorist attacks. But the conclusion is inescapable. Either the threat has been much exaggerated, or somebody must be doing something right.
Unarguably, US information-gathering has improved, especially since the appointment of John Negroponte as the country's first director of national intelligence, in overall charge of 14 previously feuding agencies. US authorities have become better at tracking the financing of terrorism. But this is a war on many fronts, so many that the question arises: is the GWOT - as Washington calls the "global war on terror" - really a war, in the normal sense of the word?
The foe, after all, is not a country or an army but a world view and an ideology, whose troops are invisible, nameless and countless. As with America's decades-old and unresolved "wars" on drugs and poverty, many of the tools for this war are not military.
Briefly last summer, Donald Rumsfeld, the Defence Secretary, and others began to talk about "the global struggle against violent extremism", in an effort to play down the military component. But within a month the new moniker had disappeared. Mr Bush underlined the point by citing the "war on terror" a dozen times in a single speech. And war it is. Events in Iraq have made sure of that.
Iraq had nothing to do with 9/11 - indeed the dislike between Saddam Hussein and Bin Laden was mutual and intense. Moreover, the 2003 invasion distracted the administration from the original goal it might otherwise have achieved in Afghanistan, the capture of Bin Laden and Zawahiri. Now, thanks to the invasion, Iraq and the war on terror have become one. Just why Mr Bush attacked Iraq remains a mystery. But the "war on terror" is the prime reason US troops are still there.
Americans may be fed up with the conflict in Iraq. Despite the administration's continuing attempts to link Saddam and 9/11, a majority now believe that pre-war Iraq was never a direct threat to the US - and that the war to remove him, in which about 2,200 US troops have died, has made them less safe. But the word "terrorism" still scares them stiff.
Even Mr Bush admits that the largest segment of the Iraqi insurgency is not foreign terrorists but ancien régime Sunnis. But that has not stopped the President, Rumsfeld and others from latching on to a part of the alleged Zawahiri letter that outlines a grand strategy of terror - starting with the establishment of an Islamic state in Iraq and then exporting this revolution to Egypt, Saudi Arabia and elsewhere. The third stage would be the destruction of the interloper, the Jewish state of Israel. Al-Qa'ida insists the letter is a CIA fake. The administration, however, brandishes it as proof that radical Islam is out to establish a new caliphate, stretching from Spain to Indonesia, governed by sharia law and casting the world back into the Middle Ages.
The chances of that happening are less than zero, but it serves the President's purpose of linkage. The Iraqi insurgents, he declared in one of four major speeches on Iraq last month, "are a direct threat to the American people". The US, he insisted, would accept nothing less than total victory. For the President, the "war on terror" started after 11 September 2001, when the US "took the fight to those who attacked us and those who shared their vision". Now, however, "the terrorists have made clear that Iraq is the central front in their war against humanity ... Bin Laden and al-Qa'ida are directly responsible for the suffering of the Iraqi people". Zarqawi and his ilk "shared the goals of the 9/11 hijackers".
But events over the next three months will be far more important than words from Mr Bush. The gamble is that last month's parliamentary election - denounced by al-Qa'ida as a "satanic project" - will lead to a government with enough authority and support to hold the country together and fend off civil war. If so, the US troop reductions yearned for by the public can take place. About 20,000 servicemen will come out early next year. If all goes reasonably well the force will be drawn down by a further 30,000 or more by the mid-term elections in November. Fewer troops should mean fewer targets for attacks, and fewer casualties.
That is Washington's strategy - or, more exactly, its hope. But, be it in Iraq or in the broader GWOT, what constitutes victory? In neither is a purely military solution possible. American soldiers, tanks and unmanned missile-carrying drones outclass anything on the other side, but in an "asymmetrical" war, they are only a part of the equation. Genuine victory will require political and societal change. It will require radical change in the Arab world's view of the US, and in Arab society itself.
The roots of terrorism lie in poverty, resentment and despair, stoked by prejudice and religious bigotry. Extremism is bred by a lack of political opportunity, a sense of inferiority to the West, and the belief the Arab world is not master of its destiny. Instead, it is subordinate to an America that, for all its talk of promoting democracy, is ultimately interested only in protecting Israel and its access to Middle Eastern oil. For that reason, whatever it says, Washington will continue to prop up the repressive regimes in Egypt and Saudi Arabia. In the last resort - as Iraq has already proved and Syria and Iran may yet prove in their turn - it will use force to have its way.
Reversing this mindset will be a colossal task. Somehow, the US and its allies must convince the Islamic world that they are not waging a religious war, or discriminating against Muslims. Unfortunately, deeds suggest otherwise.
Guantanamo Bay and Abu Ghraib, the kidnapping and "rendition" of terrorist suspects, alleged secret CIA prisons - all have created the impression that where Muslim radicals are concerned, anything goes. Nobody in a high position has been sacked for outrages that have stained America's reputation. Most astounding of all is that US leaders, self-proclaimed champions of democracy and human decency, appear to be resisting a legal ban on torture. For every terrorist captured, half a dozen potential new ones are born.
The image of the US is in tatters. The Arab world has long since made up its mind about Bush, Cheney and Rumsfeld, and their policies. No change of attitude is likely until they leave office. But the problem lies in the messengers as well as the message. America's outreach is hampered by the fact that the State Department has only 27 people fluent in Arabic - equivalent to roughly one per embassy and consulate in a region of 300 million people. September brought the bizarre spectacle of Karen Hughes, Bush's old Texas consigliera, traipsing around Arab capitals as the newly appointed head of US public diplomacy, with barely the faintest knowledge of the culture in which shehoped to create a more favourable impression of her boss.
In short, hostility to the US will persist. Americans, in all probability, will develop a more European, more fatalistic attitude to terrorism: that it is a scourge to be contained, rather than eliminated once and for all. From this perspective, the real challenge is to prevent chemical, biological and nuclear weapons falling into the wrong hands. That risk is real. The Abdul Qadeer Khan affair in Pakistan was a chilling example of how a scientist could provide nuclear technology to countries such as North Korea, Libya and Iran. If Qadeer Khan, then why not underpaid scientists from the former Soviet Union? And what about possibly unsecured Soviet nuclear and other WMD materials?
In the 2002 filmThe Sum of All Fears, terrorists detonated a nuclear bomb in a packed sports stadium in Baltimore, levelling the city centre. That was a movie, to be sure. But 11 September too, at first seemed like a disaster movie. Today reality ventures where fiction fears to tread. A nuclear attack, even in the lesser form of a "dirty bomb", on a US city is the recurring nightmare of those whose job is to keep America safe.
Whether or not Bin Laden is captured, and what happens in Iraq - even if a genuine Arab democracy takes root and every American soldier leaves - hardly affects this prospect. In the modern era, a guerrilla war lasts an average of nine years; the Iraq insurgency has been under way for less than three. As the lone superpower, perceived as malign puppet-master of the Middle East, the US will remain the target of choice for Islamic terrorists. The "war on terror" may yet acquire another name. But whatever it's called, it's here to stay.
23 Canadian federal elections
23-24 African Union summit to be held in Khartoum, Sudan
25 Elections to the legislature of the Palestinian National Authority
25-29 World Economic Forum in Davos, Switzerland
31 Retirement date of Alan Greenspan, Chairman of the Federal Reserve of the United States
5 Presidential and parliamentary elections in Costa Rica
Al-Jazeera will launch its new satellite service, al-Jazeera International, in Europe, Asia, and North America.
BBC World Service ends broadcasts in Bulgarian, Croatian, Czech, Greek, Hungarian, Kazakh, Polish, Slovak, Slovenian and Thai.
12 Parliamentary elections in Uganda
26 Parliamentary elections in Ukraine
28 Israeli general election
9 Parliamentary elections in Italy
9 Presidential and parliamentary elections in Peru
28 Presidential elections in Colombia
Vladimir Putin hosts the annual meeting of the G8 countries in St Petersburg, Russia
2 Presidential and congressional elections in Mexico
9 United States population is expected to top 300million
13-18 XVI International Aids Conference will be held in Toronto, Canada
17 Parliamentary elections in Sweden
General and presidential elections in Brazil
Nato summit takes place in Riga, Latvia
7 US mid-term elections which include: all 435 seats of the House of Representatives, 33 of the 100 seats in the Senate and gubernatorial elections in 36 statesReuse content