ON THE SAUDI-YEMENI BORDER » The five Yemeni men, all of them rail-thin, clutched their knees as they sat staring across the sand at the narrow road, which separates the Arab world’s poorest country from its richest.

"They’re waiting for us to move on," said the Saudi border guard with a weary smile, as he sat watching from the front seat of a gleaming SUV. "Waiting so they can try to cross."

This remote 1,100-mile frontier, once a casual crossing point for Bedouins and goats, has become an emblem of the increasingly global threats emanating from Yemen: fighters from al-Qaida, Shiite insurgents, drugs and arms smuggling and, well under the world’s radar, one of the largest flows of economic refugees on earth.

On some days almost 1,000 illegal migrants are caught and sent back to Yemen, Saudi officials say, including many who have come from Africa and across Yemen’s deserts fleeing war and hunger.

The porousness of the border is essential to al-Qaida’s Yemen-based branch, which has become a major terrorism concern for the United States as well as Arab countries. al-Qaida draws recruits from Saudi Arabia, where they can cross and re-cross without being noticed, and it has sent militants across to try to kill Saudi leaders in their efforts to topple the oil-rich kingdom.

In response, the Saudi authorities have embarked on a multibillion-dollar effort to strengthen the border, evacuating scores of villages that once straddled it and building elaborate defense networks to keep intruders out.

Earthen berms now prevent cars from crossing, and layers of concertina wire line the roads, some of it strewn with the rags and dried blood of desperate migrants who still try to get through. Floodlights and thermal cameras focus on different parts of the border at night, and intelligence units stand ready to interrogate anyone who is deemed suspicious.

"They adapt very quickly to every strategy we have," said Lt. Muhammad Qahtani, a seven-year veteran of the border patrol. The migrants wear their shoes backward to confuse trackers, or strap sponges to their soles to leave no footprints at all. They trek through arid mountains where the border is loosely patrolled.

Many smugglers are heavily armed and will fight to the death when surrounded, Qahtani said, because they know convicted drug traffickers are usually beheaded in Saudi Arabia.

In some ways the border here resembles the one separating the U.S. from Mexico, another desert barrier between rich and poor nations.

But this border has become far more volatile lately. A year ago Yemeni rebels killed a Saudi border guard, setting off a short war that delivered a humiliating blow to the Saudis’ well-financed but inexperienced military.

At least 133 Saudi soldiers were killed over three months, and the fighting raised alarms across the Sunni Arab world about the possibility that Iran might be supporting the Yemeni rebels – who subscribe to an offshoot of Shiite Islam known as Zaydism – and turning this border into another front for sectarian conflict.

al-Qaida’s Yemen-based branch has repeatedly boasted about its ability to infiltrate the border and outwit Saudi Arabia’s network of informants in the area. Last year, a suicide bomber crossed here and later came close to assassinating Prince Muhammad bin Nayef, who runs Saudi Arabia’s counterterrorism efforts. In October 2009, Yusef al-Shihri, a leading al-Qaida operative who had been detained at Guantanamo Bay, was killed in a gun battle after crossing the border from Yemen disguised as a woman.

Border security here involves far more than fences and patrols. Some tribes straddle the border, and they – and the Yemeni government – protested fiercely when Saudi Arabia first began reinforcing the border in 2003, saying they needed free access for grazing. That dispute seems to have eased, and the Saudi government is now refining an old policy of subsidies to border tribes with a view to security, analysts say.

"The Saudis realize they need to work with tribal leaders and make sure their livelihood depends on how good they are at keeping the border safe," said Bernard Haykel, a professor of Near Eastern Studies at Princeton who has written extensively on Yemen and Saudi Arabia. "There’s also cross-border trade, and there is a debate inside Saudi Arabia now on how hard the border should be."

In the past, many Yemenis complained that Saudi Arabia’s support for various tribal and political figures in Yemen seemed aimed at keeping their southern neighbor divided and weak. Now, as Yemen’s instability and the threat of terrorism grow worse, Saudi Arabia appears to be reassessing its approach to Yemen and its longtime president, Ali Abdullah Saleh, diplomats say.

"They are trying to be more systematic," said a Western diplomat in the Saudi capital, Riyadh. "Their manipulations are now aimed at supporting Saleh, because he’s the only game in town."

The border was officially demarcated only in 2000. Much of it remained so informal that many villages on the border’s western edge, near the Red Sea, were half Yemeni, half Saudi. Those days ended last year with the war, when the Saudi government evacuated 78 border villages and extended the network of fences it had begun building several years earlier.

The area is an eerie wasteland now – scores of houses, some of them pockmarked with bullets from the war, sit empty and silent. At the top of the mountain where the fighting started last year, Saudi soldiers man a .50-caliber machine gun, gazing across at the unmarked ridges that form the border with Yemen.

Inside the border patrol headquarters in the port city of Jazan, photographs line the wall showing contraband captured by the patrol guards: truckloads of rocket-propelled grenades, huge bricks of hashish, stacks of machine guns.

Drug smuggling has risen by almost a third in the past two years, Saudi officials in Jazan say, with more than 7,000 pounds of hashish seized so far this year. The most dangerous smugglers of all are those who drive through the Empty Quarter, the Texas-size sand desert that dominates the southern part of the Arabian Peninsula, patrol officers say.

But far more numerous are the illegal migrants, hundreds of thousands of them annually in recent years. Most are caught and sent back to Yemen after being held in crowded border detention centers for a day or so. Many have crossed the sea to Yemen from Somalia or Ethiopia, risking death on rickety boats in shark-infested waters. Most of the survivors make the arduous journey through Yemen’s arid mountains only to be turned back at the Saudi border.

"Some of them say, ‘If you give me something to eat, I will go back,’ said Qahtani, the border patrol officer. "You can only feel pity for these men."

© 2010 The New York Times Company