Christopher Walker: The shadow of al-Qaida falls across Yemen

They were responding to a threat from the recently formed group

al-Qaida in the Arab Peninsula (AQAP), a merger between al-Qaida cells in Yemen and Saudi Arabia, to kill Westerners and attack Western targets in the capital, Saana.

The group, estimated to have some 300 members, used the internet to declare "all-out war against every crusader on Mohammad's Peninsula".

The belligerent message was clear proof that the fanatical Islamic group with bases in remote mountain areas of the country had been emboldened by the failed attempt by a Yemen-trained Nigerian suicide bomber, Umar Farouq Abdulmutallab, to blow up a Detroit-bound US airliner on Christmas Day.

The Western diplomats had good reason to take the call "to every Muslim who cares about his religion and doctrine to assist in expelling the apostates from the Arabian Peninsula by killing every crusader who works at their embassies" seriously.

Only in December, Yemen announced that it killed four members of an AQAP cell that was plotting to blow up the British Embassy. In 2008, two years after 23 convicted and suspected terrorists escaped from a high-security jail in the city, the US embassy was the target of an attack in which 19 people died.

In moves eerily reminiscent of the beginnings of the Vietnam War, trainers from the US have been working discreetly with Yemen's elite counter-terrorism unit for several years. But, during the past 12 months, concern that al-Qaida could establish a new "safe haven" in the more ungovernable areas of the Arabian Peninsula have been growing.

Washington began to step up military and security aid with about $70m spent last year alone. At the weekend, General David H Petreus, the regional US military governor, flew in with a pledge to double this figure in 2010 and a personal message from President Barack Obama to President Ali Abdullah Saleh.

But the Americans have been warned that any direct intervention will inflame the inherent anti-Americanism among many ordinary Yemenis, whose observant Muslim nation is situated only a few hundred miles from Islam's holiest site, Mecca.

The modern Republic of Yemen was only created in 1990 when traditionalist North Yemen and Marxist South Yemen merged after years of border wars and skirmishes. The peace, in fact, collapsed briefly in 1994, and ever since, many in what they consider is the under-privileged south have been resentful. This has left a weakened and ageing President Saleh facing a tribal rebellion in the north and increasingly violent separatist sentiment in the south.

With diminishing oil reserves, he has been unable to pay off tribesmen in remote desert and mountainous areas. They have inevitably turned to al-Qaida for financial patronage – an alliance reinforced by the fact that Yemen was the ancestral homeland of Osama bin Laden.

Although the Yemen government is weak and unpopular, President Obama has little choice but to work through it. He has now been joined by Gordon Brown, who has called an emergency international conference on January 28 to discuss strategy.

Regional crackdowns in places like Saudi Arabia, Iraq and Pakistan have encouraged al-Qaida sympathisers to flee to Yemen's remote areas, where new camps have been established.

"Before last year's merger, AQAP was a fairly weak, loosely knit group," said Saeed ali-al Jamhi, author of a well-informed book on al-Qaida's activities in Yemen. "Today, it is a fully-fledged organisation. The question is, could it be destroyed? It could not. At best, it could be contained."

The rise and rise of al-Qaida in the country where back in 2000, the Navy destroyer USS Cole was attacked near the port of Aden with the loss of 17 US lives, has been assisted by new leaders released from the US prison at Guantanamo Bay and the influx of some 200,000 refugees from Somalia.

The nightmare possibility now haunting Western strategic planners is that Yemen will follow Somalia into becoming a failed state, thus automatically strengthening the dangerous foothold that al-Qaida has already established there on the borders of the world's largest oil producer, Saudi Arabia.

Christopher Walker is a former Middle East Correspondent of The Times.