CAIRO - Even in the best of times, Yemen looks like a nation about to unravel.

Now that the U.S.-allied leader has left Yemen for medical treatment and may not return, citizens of the poorest Arab country are contemplating a future perhaps even worse than the 33 years under authoritarian rule.

The question of who would ultimately replace President Ali Abdullah Saleh could unleash new and unpredictable power struggles among the country's powerful tribes, the youth movement that has led the anti-Saleh protests, and remnants of the leader's regime, including his son.

In the meantime, the numerous conflicts and economic and social problems that were already leading Yemen to ever-greater disorder and hardship before this year's unrest broke out look certain to remain unaddressed as the political crisis deepens.

All that is of great concern far beyond Yemen's borders, especially for the United States, which had relied on Saleh in battling one of the most active branches of al-Qaeda.

Al-Qaeda in the Arabian Peninsula has an estimated 300 hard-core members taking sanctuary in Yemen's forbidding mountains, plotting attacks on targets in Europe and the United States while enjoying the protection of tribal chiefs at odds with Saleh's regime.

Al-Qaeda militants, who have been linked to several nearly successful attacks on U.S. targets, would likely benefit from more chaos, gaining more freedom to plot attacks without being disrupted by Saleh's U.S.-trained counterterrorism forces. But they do not have the manpower to capture and hold any significant part of Yemen.

Yemen is also home to U.S.-born Muslim cleric Anwar al-Awlaki, whom the United States has on a kill-or-capture list.

And other destabilizing threats could now worsen, including a secessionist movement in the south that has steadily gained strength in recent years, raising the specter of Yemen breaking into two parts - or a civil war to prevent that from happening.

Saleh has used a mixture of political acumen, patronage, and violence to keep Yemen together, if only just, and will be a hard act to follow in a country where almost every adult male has a firearm, religious fundamentalism is widespread, and resentment, even contempt, toward authority is a popular sentiment.

No one in Saleh's regime has enough of his qualities to succeed, said Rick Nelson, a counterterrorism expert at Washington's Center for Strategic and International Studies: "I can't see any remnant of the Saleh government staying in place after this."

Saleh's departure may have spared the country civil war, but the standoff between him and protesters across the nation who demand his immediate resignation has laid bare Yemen's divisions and compounded its economic woes.

Shrouded in secrecy, Saleh left for Saudi Arabia early Sunday to seek medical treatment for wounds he suffered Friday in a rocket attack on his compound in the capital, San'a. News of his departure sparked wild celebrations on the streets of San'a, but the fear of what could be in store for Yemen was not far from the mind of some.

"We are happy he is gone," protester Fayza Abdullah said. "Let us celebrate his departure regardless of what the future brings."

The tribesmen, a powerful and conservative force, would likely take credit for ousting Saleh and seek to dominate the next government if the president never comes back. That would in turn place them at odds with the mostly liberal youth groups that have organized the massive anti-Saleh protests since February. The groups also fear that the top army officers who defected to their side in March would eventually abandon them.

Both tribal leader Sadeq al-Ahmar and most senior officers in Yemen benefit from the generosity of the Saudis.

An additional source of danger comes from the powerful members of Saleh's regime whom the president left behind in Yemen, including his son and heir apparent Ahmed, who heads the well-equipped and highly trained Republican Guard, and four close relatives who command key military units.

"I fear the battle of the desperates," human-rights activist Majed al-Madhaji said, alluding to what Saleh's relatives could do. "This is the biggest fear, that they decide to bring down the temple."

Yemen, meanwhile, has been inching closer to economic collapse.

Already, the country is running out of water, and the little oil it produces has stopped because of the upheaval. Oil accounts for about 60 percent of the government's revenue, and more than 90 percent of what it collects from exports. The drop in oil revenue has left Yemen facing multibillion-dollar annual budget deficits in recent years.

Nearly half of all Yemenis live under the poverty line set by the World Bank at $2 per day and do not have access to proper sanitation. Less than a tenth of the country's roads are paved; more than half the children are malnourished.

The unrest of the last three months has led to a dramatic surge in food prices in the capital and a shortage of basic items, forcing tens of thousands to flee to rural areas where prices remain affordable.