As US soldiers gird for an airborne assault on Osama Bin Laden's strongholds in Afghanistan, another army has set about breaching the Saudi renegade's defenses on another front - his finances.
The terrain that this global alliance of forensic accountants, banking experts, and spies must reconnoiter is, in many ways, as forbidding as the mountainous hinterland of Afghanistan. A maze of front organizations, shell companies, and underground banking networks shields the money from view, say investigators.
Already, experts are questioning whether that shield can be torn down without stifling the freedom of financial flows that underpins today's world economy - or without challenging the authorities in countries such as Saudi Arabia that appear to shelter terrorist financiers - a diplomatically delicate task.
"It is hard to imagine how Al Qaeda's financial circuits can be completely frozen," says Jean-Luc Marret, a terrorism expert at the Strategic Research Foundation, a Paris-based think tank.
Saudi Arabia and its five partners in the Gulf Cooperation Council have agreed to block funds belonging to people on two US lists of suspected terrorists and those suspected of financing terrorism, but have not yet found any suspicious money, according to Bahrain's finance minister, Abdullah Saif. That, despite the fact that Yasin al-Qadi figures on the second list. Mr. al-Qadi is a prominent Saudi businessman who headed the Muwafaq foundation, which the US Treasury Department described as "an Al-Qaeda front that receives funding from wealthy Saudi businessmen."
The nature of Saudi funding for Mr. bin Laden's network is suggested by the fact that Prince Mohammed al Faisal al Saud - a cousin of King Fahd - heads the Dar al Maal al Islami bank, which joined bin Laden in founding the Al Shamal bank in Sudan, according to a report prepared for the French intelligence service.
Bin Laden used the al-Shamal bank to channel funds to his terrorist network, according to testimony by a former bin Laden business associate, Jamal Ahmed Mohammed al-Fadl, during the trial of Al Qaeda agents for the 1998 bombing of two US embassies in Africa.
Given Saudi Arabia's status as a key US ally in the Middle East, it will clearly be hard for Washington to confront the government there forcibly enough to make it act on such information.
"Saudi and Gulf banks were under-represented on the list of institutions whose assets are to be frozen," says Mr. Marret. "The lists were drawn up on the basis of intelligence information, but also with foreign policy considerations in mind."
"The names on the lists so far have been those of foot soldiers," adds Guillaume Dasquie, editor of the French magazine Intelligence World, who has close ties to Western intelligence agencies. "It will only make a difference if they target the commanders. But it is much harder, and much more political, to block the original sources of the money."
So far, 102 countries have promised to freeze alleged terrorist assets, and 62 governments have actually adopted blocking mechanisms, according to the US Treasury.
By executive order, President Bush has frozen the US assets of a total of 66 individuals, businesses, and charities that are believed to have channeled money to Bin Laden, chief suspect in the Sept. 11 attacks on New York and Washington.
Such moves are a top priority for Western governments joining in the war on terrorism. "Those who finance terror are as guilty as those who commit it," says British Finance Minister Gordon Brown this week, introducing new measures to crack down on money laundering.
"Our response to the funding of terrorist acts must be every bit as clear, as unequivocal, and as united as our response to the terrorist acts themselves," he told Parliament. "If fanaticism is the heart of modern terrorism, finance is its lifeblood."
That opinion is not universally shared, however. "Choking off the funds is not the real issue," argues Alex Standish, editor of the authoritative Jane's Intelligence Weekly. "It is erroneous to think that terrorist acts are very expensive - most terrorism is very low-budget."
Planning, preparing and carrying out the Sept. 11 attacks, he estimates, cost only a few hundred thousand dollars.
Tracking those sorts of sums is especially complicated if they are moved through an informal banking network known as hawala, which is commonly used throughout Asia.
Under the centuries-old system, which is almost paperless, an Asian worker who wants to remit money to his family back home simply hands the cash to a hawala banker, who faxes or phones a colleague in Asia with the code word that the worker's relative will use to collect the money. Hawala bankers settle their debts among themselves.
When it comes to legitimate, above-ground channels, bin Laden and other terrorist groups have been able to exploit the international financial system, with its easy and almost instantaneous capital flows.
It can take less than a day to move hundreds of thousands of dollars around the world, between several bank accounts in a financial system that has made globalization possible. International businesses depend on such ease of movement, and any steps to restrict it would meet with stiff resistance.
"There is a limit to the fight against terrorist financing, and that is the freedom of business," says Mr. Marret. "At a certain point, they are in contradiction with each other."
Until Sept. 11, for example, Washington opposed international efforts to clamp down on tax havens, where financial controls are weak, despite evidence that criminals take advantage of them. US officials argued that tighter controls would restrain legitimate competition among countries to attract capital.
European guarantees of the free movement of capital are also an obstacle to Washington's call for asset freezing. Though some individual countries have blocked funds belonging to people or institutions on the US lists, the European Union has no power to ban the movement of capital among its members unless there is a threat from a third country, which is not currently the case.
"We would like to act quickly alongside the United States, but at the moment it is not always possible," says Gunnar Wiegand, spokesman for the EU External Affairs commissioner Chris Patten. Mr. Patten is urging new EU legislation that would allow "things like this to be adopted immediately, at the same time as in the United States," so that suspected terrorists do not have time to move their funds, Mr. Wiegand adds.