Even at the best of times, Yemen looks like a nation about to unravel.

Now that the US-allied president has left the country for medical treatment and may not return, citizens of the poorest Arab country are contemplating a future that could be even worse than the last 33 years under authoritarian rule.

The question of who would ultimately replace 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 this is of great concern far beyond Yemen’s borders, especially by the US government, 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 would likely benefit from more chaos in Yemen, gaining more freedom to plot attacks without being disrupted by Saleh’s US-trained counterterrorism forces. But the militants don’t have the manpower to capture and hold any significant part of Yemen or replace government authorities.

However, the Saudis and their American backers would want any future regime in Yemen to forcefully go after al-Qaeda militants to disrupt their capabilities.

The group has been linked to several nearly successful attacks on US targets, including the plot to bomb a Detroit-bound airliner in December 2009. It also put sophisticated bombs into US-addressed parcels that made it onto cargo flights.

Yemen is also home to US-born Muslim cleric Anwar al-Awlaki, whom the United States has put on a kill-or-capture list. Washington accuses him of becoming a key al-Qaeda operative and inspiring attacks on the US, including the 2009 shooting at a military base in Fort Hood, Texas, that killed 13 people.

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

The north is home to rebellious Shiites who the government insists are backed by Iran.

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, of authority is a popular sentiment.