That move proved to be a turning point, one that radicalized Al-Shabaab, which attacked Ethiopian forces and gained control of parts of central and southern Somalia, according to a 2011 case study by Rob Wise, who was then with the Counterterrorism Program at the Center for Strategic and International Studies.
What is Al-Shabaab's relationship with Kenya?
In 2011, the Kenyan government ordered a cross-border incursion intended to create a security buffer zone in southern Somalia after attacks on tourist destinations in northern Kenya that it blamed on Al-Shabaab. More recently, Ethiopian troops crossed the border and expelled Al-Shabaab from Baidoa, a strategic town midway between the Ethiopian border and Mogadishu.
The group then targeted African Union soldiers and government buildings in the capital in suicide attacks. A suicide bombing in March 2012 killed five people at the Presidential Palace.
Analysts say tension appears to have been growing within Al-Shabaab between Somalis and foreign fighters, several hundred of whom are thought to have entered Somalia in recent years to join the group.
There may also have been disagreement within the group about the announcement in February 2012 of an alliance between Al-Shabaab and al Qaeda and about the group's ban on foreign aid organizations working in Somalia to save millions threatened by famine.
How and from where does it recruit?
The organization has a sophisticated public relations arm that includes a Twitter account and video production abilities.
"Remember Mumbai?" one tweet asked Saturday, as gunfire was erupting from Westgate mall in Nairobi. The comment was an apparent reference to the 2008 attack in which 10 Pakistani men associated with the terrorist group Lashkar-e-Tayyiba stormed several buildings in Mumbai, India, killing 164 people over a three-day period.
"Some youngsters resist death even when told not to be taken alive. It's going to be a long ordeal," Saturday's tweet said.
Soon after, it announced it was responsible for Saturday's attack in Nairobi, Kenya. "Alshabab confirms its behind the #Westgate spectacle," it said.
A 2009 Al-Shabaab video is as slickly produced as a reality TV show, complete with a hip-hop jihad voice and a startling message.
"Mortar by mortar, shell by shell, only going to stop when I send them to hell," an unidentified voice raps on the video in American English.
The video shows a man reported to have been Abu Mansoor al-Amriki, a U.S. citizen from Alabama. "Away from your family, away from our friends, away from ice, candy bars, all those things is because we're waiting to meet the enemy," he says.
But enemies -- and alliances -- can shift.
Al-Amriki, whose real name is Omar Hammami, said last year in a video posted online that he had had a fallout with Al-Shabaab "regarding matters of the sharia and matters of strategy" and feared for his life. He was reported last week to have been killed in Somalia by Al-Shabaab. CNN is not able to confirm the report.
Finding replacements may not be hard.
Sheikh Ahmed Matan, a member of Britain's Somali community, said he knows of hundreds of young Somali men living in the West who returned to Somalia for terrorist training.
How is it funded?
The once ragtag Somalia-based al Qaeda affiliate has grown into an economic powerhouse, raising tens of millions of dollars in cash from schemes that have involved extortion, illegal taxation and other "fees," according to the 2011 United Nations report.