Some of al Qaeda's most effective operators, armed with its most lethal weapons, are based in Yemen, a failing state in the southwestern corner of the Arabian Peninsula, where most people believe the world's most dangerous terrorist movement is a figment of U.S. propaganda. No one knows Yemen better than Saudi Arabia's intelligence service. Its agents in the capital of Sana'a were the first to alert U.S., British and German intelligence about bombs on their way to the United States disguised as harmless United Parcel Service packages designed to detonate in midair. In case that failed, they were addressed to synagogues in the Chicago area, where they would detonate when opened.

North and South Yemen seesawed between one and two states, between Marxism and feudalism, after Britain's last hurrah in Aden (1967) when local police mutinied, killed 24 British troopers and dragged the mutilated bodies through the streets. In a last gasp of empire, Col. Colin Mitchell - known to reporters as Col. Mad Mitch - charged rebels in Crater Harbor with 15 regimental bagpipers playing "Scotland the Brave." Then the Argyll and Sutherland Highlanders flew home, only to be replaced by a Soviet client state - the People's Democratic Republic of Yemen.

North Yemen, meanwhile, opted for royalty, mostly to curry favor with big royalist brother next door in the kingdom of Saudi Arabia. Egyptian dictator Gamal Abdel Nasser detected an opportunity for his Pan-Arab dream by knocking off the Yemeni royals and undermining the House of Saud. A regiment of special forces and a wing of fighter bombers, Nasser reckoned, would secure a Yemeni republic. A good-money-after-bad escalation followed, first to 15,000 regular troops in 1962, 36,000 in 1963, 50,000 in 1964, all the way up an Egyptian expeditionary corps of 70,000 in 1965 - made up of 13 infantry regiments, one artillery division, one tank division and most of Egypt's special-forces and paratroop regiments.

Saudi Arabia and its local Yemeni royalist proxies inflicted heavy casualties on the Egyptians - some 10,000 were killed - and Nasser, the geopolitical gambler, brought his tattered army home only to double down by remilitarizing the demilitarized Sinai Peninsula, closing the Straits of Tiran to Israeli shipping, blockading the port of Eilat, Israel's access to the Indian Ocean - only to face yet another military defeat. In the first two hours of the 1967 Six-Day War, Egypt lost more than 300 Egyptian warplanes, and it lost 451 by war's end. Nasser's army suffered its second humiliating defeat in two years.

Overly confident after its successful revolution and ouster of the British imperial power, South Yemen's Marxist regime, advised by East Germany's Stasi spy chief Markus Wolf, also pushed its luck by sponsoring the Marxist-dominated Popular Front for the Liberation of Oman and the Arab Gulf. Advised by British officers seconded to their forces, the Omanis got Yemen's proxies to cry Arabic for uncle, and the entrance to the oil-rich Gulf remained in Omani hands.

Officially reunified, North and South Yemen have been under the presidency of Ali Abdullah Saleh for two decades. Before that, Mr. Saleh had been president of North Yemen since 1978.

In 1990, Yemen under Mr. Saleh was the only country - apart from Cuba - to oppose the Iraq War. The Saudis made him pay dearly for his unruly behavior. They expelled more than a million Yemeni workers who had enjoyed a special status that had entitled them to work in the kingdom. This made Yemen's economic crisis that much more acute.

A Yemeni Houthi armed resistance group in the north that is anti-Saudi and anti-Saleh (and some claim is supported by Iran) and new separatists in South Yemen give the central government little sway beyond the capital's city limits. The majority of a population of 23 million is younger than 30, almost half of them unemployed. Al Qaeda's siren song on the Internet, even in remote villages, is that the enemy is America.

Most Yemeni males chew qat, a mild narcotic, several times a day. But qat consumes more water in cultivation than any other plant. Half the available water for irrigation goes to qat. Water shortages are acute throughout much of the Middle East and even more so in Yemen. There is only enough water for half of Sana'a 2 million residents. Some experts think Sana'a may be the first capital city to die of thirst.

As Yemenis survey the scene in Iraq, Afghanistan and Pakistan, it does not require an overwhelming effort of imagination to conclude, with the assistance of al Qaeda's operatives, that the United States is at war against Muslims. Mr. Saleh is seen by many as an American puppet who authorized the Saudis to bomb northern Houthi villages.

There is no shortage of recruits for al Qaeda's religious proselytizers, who chant the merits of jihad. And it is hard to distinguish between angry unemployed youngsters with easy access to arms and al Qaeda's fighters. Those familiar with both Yemen and Saudi Arabia since Sept. 11, 2001, say that al Qaeda sympathizers in the Saudi kingdom were carrying out attacks in Saudi cities until 2003, when opinion turned against jihadis. By 2006, pro-jihadi resistance had been crushed. And those captured were put through intensive de-brainwashing courses.

This is not likely to happen in Yemen, where, unlike in Saudi Arabia, extreme poverty keeps the jihadi pot boiling. Poor Yemenis tend to say that so-called al Qaeda killings are organized by Mr. Saleh's government as a pretext to crack down on anyone who speaks too loudly against authority.

Arnaud de Borchgrave is editor-at-large of The Washington Times and of United Press International.