What use are warnings with an enemy that is so unpredictable?

Al-Qa'ida has suffered numerous setbacks since September 2001. It has been driven out of its Afghan hideouts and several of its most senior figures have been killed or arrested. Yet the network has once again shown its ability to strike back, venomously and unpredictably.

Al-Qa'ida has suffered numerous setbacks since September 2001. It has been driven out of its Afghan hideouts and several of its most senior figures have been killed or arrested. Yet the network has once again shown its ability to strike back, venomously and unpredictably.

Despite a claim of responsibility by a previously unheard-of group in Lebanon, and leads pointing to Somalia, nobody doubts that Osama bin Laden's terror network lies behind Thursday's attacks in Kenya, in which 16 people, including three Israelis and three suicide bombers, died.

The near-simultaneous attempts to kill Israeli tourists, at the Paradise beach hotel near Mombasa and on an airliner leaving for Tel Aviv, is an al-Qa'ida trademark, echoing both the September 2001 strikes on New York and Washington and the 1988 bombings, only minutes apart, of the US embassies in Kenya and Tanzania. Only slight errors – in the timing of the suicide bombing at the Paradise, and in aiming the surface-to-air missiles fired at the airliner – prevented last week's death toll far exceeding that in Bali, where more than 200 people died on 12 October. But the latest atrocity succeeded in demonstrating al-Qa'ida's reach.

Since the attacks on the World Trade Centre and the Pentagon, the network has struck in south-east Asia (Bali), south Asia (Pakistan), the Middle East (Yemen, Kuwait and Jordan) and north Africa (Tunisia). Now it has returned to east Africa, the scene of its first international "spectacular" in 1998.

A pattern is beginning to emerge, according to intelligence sources and analysts of terrorism. While it rebuilds its organisation, financing and communications out of sight of the West, where counter-intelligence agencies have been galvanised by the 11 September horror, al-Qa'ida is concentrating on targets in parts of the world where poverty and political instability are common, security services are less sophisticated and anti-Western sentiment is high.

"The security environment is tougher in Europe, where about 160 suspects are detained, so they are moving to softer targets," said Magnus Ranstorp, deputy director of the Institute for the Study of Terrorism at the University of St Andrews. Jonathan Stevenson, terrorism expert at the International Institute of Strategic Studies in London, agreed. "Al-Qa'ida is bringing forward operations in areas where it has partners, such as Jemaah Islamiyah in south-east Asia, or where it has an existing cell structure, as it still does in Kenya," he said.

Somalia, the "failed state" where US investigators believe the Mombasa attacks were planned, is as ideal an operating environment for the network as Afghanistan once was. (The Americans learnt that to their cost in 1993, when they lost 18 troops in one day during a disastrous peacekeeping mission, as recounted in the book and film Black Hawk Down.) The Soviet-made Strela missiles fired at the Israeli charter airliner had almost certainly been in Somalia for years, said one source. And al-Qa'ida did not merely have a partner in the country – a group called al-Ittihad al-Islami – it had actually created it, and organised the arming and training of its members.

With these advantages, "soft targets" such as the Paradise hotel and Mombasa airport had little chance. While Western nations struggle to protect their embassies and other national symbols abroad, they can hardly guess where the terrorists will strike next. "It's impossible to guard against something like this," Vince Cannistraro, a former head of the CIA's counter-terrorism division, told The New York Times of the Kenya attacks. "There are targets all over the world, and tourists are totally defenceless."

Al-Qa'ida has frequently threatened Israel, but scarcely ever penetrated its security. This attack makes good on its rhetoric. It also answers some critics in the Arab world who accuse the network of ignoring the Palestinian struggle in its single-minded focus on Western, Christian targets. That in turn will help its recruitment efforts in the Middle East as war with Iraq looms – a rich return for one "soft" operation, even if it raises doubts about al-Qa'ida's ability to strike in Israel itself, or in the West.

But the network's resilience has been evident since it first began activities in the early 1990s. Before and after the embassy bombings in Nairobi and Dar es Salaam, numerous plots failed, including one to blow up American airliners over the Pacific and an attempted attack on Los Angeles airport during the millen- nium celebrations. In September 2001, three years after the east African attacks, al-Qa'ida succeeded dramatically.

No successful terrorist operation has since been carried out in the West, though the British "shoe bomber", Richard Reid, came close last December. One source said, however: "You have to remember that these people think long-term. It is only 14 months since they hit America. In the meantime they are reminding the world that they are still around, and still prepared to inflict mass civilian casualties."

Mr Stevenson believes that al-Qa'ida's ability to develop several plots at once has probably diminished since the Afghan campaign, when it lost senior operatives such as Mohammed Atef, its military chief. But he added: "It has managed to reconstitute the remaining members of its inner circle and bring in members of the second generation, even if some of them are still learning. They are coalescing outside Afghanistan, mainly in Karachi."

Dr Ranstorp said: "Their operational innovation is remarkable. The variations [in their operations] make them very unpredictable, a security nightmare. They use local groups for reconnaissance, who then go back to the centre if they need resources." Against this background, security experts say, the debate about possible warnings which begins after every act of terrorism is beside the point.

It was reported in the wake of last week's attacks that Australia had specifically told its citizens to stay away from Mombasa, setting off a row over why Britain and the US had failed to issue similar warnings. But both the Foreign Secretary, Jack Straw, and American officials say they received the same information, and that it was not, in the words of the US side, "credible and specific" enough to predict what was going to happen.

The scale of the problem was illustrated by a US State Department official recently, who estimated he had analysed about 9,000 terrorist threats over a 25-year career. "I can only think of about two dozen reports which provided all the key pieces of a credible terrorist plot," he said. "I think it is crucial that people understand the quest of detecting key pieces of a terrorist plot is extremely difficult."

Australia was reacting strongly to every reported threat since the Bali bombings killed scores of its citizens, said sources, but as Tony Blair pointed out recently, such reports flood in by the hour. Hardly ever is there enough information to avert an imminent attack; to issue a stream of warnings not only devalues them, as the US discovered earlier this year, it risks doing the terrorists' work by bringing daily life to a standstill.

But if Westerners can hardly refuse to leave their homes in case of terrorism, they may have to think about where they go abroad.

In retrospect, it made no sense to go to Bali, since Indonesia is the most populous Muslim country in the world and among the least stable. Israelis were equally unwise to congregate in a fiercely Islamic part of Kenya where the 1988 Nairobi bomb is known to have been made. In future it will not be enough to dream of winter sun and exotic locales without weighing the risk of never returning.