The United States believed then that the group was coordinating with al Qaeda groups in Yemen and might have been plotting attacks in the region and abroad.
In 2011, it was generating "between $70 million and $100 million per year, from duties and fees levied at airports and seaports, taxes on goods and services, taxes in kind on domestic produce, 'jihad contributions,' checkpoints and various forms of extortion justified in terms of religious obligation," according to the report from the U.N. Monitoring Group on Somalia and Eritrea.
How have Somalis been affected?
Years of lawlessness and poverty have exacted a toll that Al-Shabaab has not helped. In 2011, the United Nations declared a famine in the southern Somalia regions of Bakool and Lower Shabelle, and Al-Shabaab reversed an earlier pledge to allow aid agencies to provide food in famine-stricken areas.
That year, the U.N. Interagency Group for Child Mortality Estimation announced that Somalia had the highest mortality rate for children 4 and younger in the world.
In May, a report jointly commissioned by the U.N. Food and Agriculture Organization and the USAID-funded Famine Early Warning Systems Network reported that 258,000 Somalis had died in the famine between October 2010 and April 2012 and that half of the victims were younger than five.
What is the United States doing?
The United States has supported U.N.-backed African forces fighting Al-Shabaab and strengthened its counterterrorism efforts against the group.
It has also donated millions in aid.
The U.S. State Department said this week that Somali security forces, aided by the African Union Mission in Somalia, have driven Al-Shabaab out of major cities and towns, creating "a window of opportunity to fundamentally change Somalia's trajectory.
What is the status of Somalia's government today?
In September 2012, Somali parliament members selected Hassan Sheikh Mohamud as the new president in a vote that marked a milestone for the nation, which had not had a stable central government since Barre's overthrow 21 years before.
That did not mean Al-Shabaab was calling it quits. In January, French forces attempted to rescue a French intelligence commando held hostage in Somalia by the group. The raid left the soldier dead, another soldier missing and 17 Islamist fighters dead.
But there has been political progress there.
In January, for the first time in more than two decades, the United States granted official recognition to the Somali government.