Al-Shabab power struggle behind attack in Nairobi

The attack on the Westgate shopping centre in Nairobi by Islamist militants from the Somali-based al-Shabab group is a direct product of the long-running failure of western powers and African Union countries to end more than 20 years of anarchy in the "failed state" of Somalia.

But it also reflects the outcome of a brutal power struggle within al-Shabab that has brought the group's hardline global jihadist wing to the fore.

Video: Volunteers help soldiers and police take on Islamist militants

At first glance the Westgate atrocity simply looks like a vicious reprisal for successful military operations undertaken in southern Somalia by the 4,000 Kenyan troops attached to Amisom, the 18,000-strong African-Union-led, UN-backed peacemaking mission. A statement by al-Shabab said as much, and threatened more of the same until the "Kenyan invaders" withdrew. But Westgate can also be seen as a chilling statement of intent by Ahmed Abdi Godane, the al-Shabab leader, who consolidated his power in June in an internal coup.

Among four top commanders who were executed by Godane were two of the group's co-founders, known as al-Afghani and Burhan. Al-Shabab's spiritual leader, Sheikh Hassan Dahir Aweys, fled for his life, and was subsequently detained by Somali government forces.

The infighting continues. Earlier this month, the Alabama-born al-Shabab commander Omar Hammami, known as Abu Mansoor al-Amriki or "the American", and a British national known as Usama al-Britani, were shot dead in a dawn raid on their hideout by Godane's allies.

Hammami, who was on Washington's "most wanted" list, had previously accused Godane of behaving like a dictator.

Godane, also known as Mukhtar Abu Zubayr, was behind al-Shabab's decision in 2011 to affiliate to al-Qaeda and adopt its global jihadist outlook. It is Godane who is said to have ordered the 2010 bombings in Kampala that killed 74 people - in protest at Uganda's participation in Amisom. In 2011 he published a jihadist video entitled At your service, Osama. In it he vowed that "the wars will not end until sharia [law] is implemented in all continents in the world".

Even before Westgate, he was one of the world's most wanted terrorists, with a US$7 million bounty on his head. Sheikh Aweys, in contrast, is seen as a Somali Islamist nationalist opposed to foreign intervention of any kind, be it jihadist, Western or African, a position he elaborated in a Guardian interview in 2008.

His vanquishing was a victory for the hardliners, who are now in the ascendant. "[They] will want to show that it [al-Shabab] remains a cohesive force, and my fear is that there will be an escalation of conflict, with more bombings," the Kenya-based Somali analyst Rashid Abdi presciently told the BBC after the June coup.

Al-Shabab is under pressure on a number of other fronts.

Having been ejected from the Somali capital, Mogadishu, two years ago, it is facing a renewed campaign to retake key central towns. Last week the central town of Mahadeey was overrun by Somali troops backed by Amisom. Although it still controls much of the country's south, loss of territory means loss of revenue and influence for the group.

Meanwhile, 150 leading clerics have signed a government-supported fatwa asserting al-Shabab under Godane has strayed from Islam's true path.

The apparent decision by Godane and fellow hardliners to again take the fight beyond Somalia's borders looks like a bid to regain initiative in the face of its setbacks and disagreements.

Additional reporting by Agence France-Presse