The buildup of the AU force, known as Amisom, could end up stoking violence rather than lessening it because it's likely the reinforcements will include troops from Ethiopia, widely reviled across the Horn of Africa country for its U.S.-backed invasion in 2006 that toppled a short-lived Islamist government.
U.N. Secretary-General Ban Ki-moon warned in October "military gains against al-Shabaab that have been achieve in recent years are at serious risk of being reversed" as the Islamists regroup and mount large-scale attacks, including suicide bombings in Somalia's war-battered capital Mogadishu.
Al-Shabaab -- it means "the Youth" in Arabic -- was driven out of Mogadishu and other major Somali cities over the last two years by Amisom, heavily reinforced with Kenyan and Ethiopian forces, but it retains control of large areas of the countryside.
The International Institute for Strategic Studies in London estimates al-Shabaab currently comprises 7,000-9,000 active fighters, "a significant reduction from a May 2011 estimated of 15,000.
The movement is controlled by hard-line jihadists, led by Afghan War veteran Ahmed Abdi Godane after an internal power struggle.
It's showing no sign of scaling back its operations, despite an abortive U.S. Navy SEALS attempt in October to capture top al-Shabaab leader Abdulkadir Mohamed Abdulkadir, a Kenyan of Somali origin known as "Ikrima."
It's also widened its war against Kenya, Uganda, Ethiopia and other regional African states that contribute troops to Amisom, which has not been a peacekeeping force for some time but a strike force tasked with crushing the Islamist alliance of Somali clans.
These groups have been fighting each other in an ever-changing sequence of tribal alliances since Somali warlords toppled the dictatorship of Gen. Mohamed Said Barre in 1991, the last fully functioning central government the country has had.
The poorly led military forces of the transitional federal government of recently elected President Hassan Sheik Mohamud are unable to cope with al-Shabaab on their own and are totally dependent on the AU force and its firepower.
Al-Shabaab has unleashed a campaign of guerrilla warfare, avoiding set-piece battles with Amisom, and widened its campaign with bombings in the countries of Mohamud's African allies.
The most savage of these was the Sept. 21 attack by a four-man cell on the Westgate shopping mall in Nairobi, Kenya's capital, in which as many as 94 people, including the attackers, are reported to have been killed during a four-day siege.
In Ethiopia, like Kenya a largely Christian country, four people were killed Nov. 6 when a bomb exploded in a minibus in the western Segno Gebaya region, possibly as they were transporting a bomb.
Ethiopian security authorities said two weeks earlier that two Somali suicide bombers accidentally blew themselves up in a house in Addis Ababa, the country's capital, while preparing to attack soccer fans during Ethiopia's World Cup qualifying match against Nigeria.
Ethiopia has been plagued by terrorist attacks by insurgent groups such as the Oromo Liberation Front and the Ogaden National Liberation Front for years, but none of the factions have employed suicide attacks.
If the Ethiopian account is accurate, the plot bore an eerie similarity to al-Shabaab's deadly attack on Kampala, Uganda's capital, July 11, 2010, in which 74 people were slaughtered in coordinated bombings against crowds watching the World Cup final on giant satellite TV screens.
These and other attacks have raised the specter of Christian-Muslim conflict across East Africa. Sectarian violence has recently been reported in Tanzania as al-Qaida groups and their offshoots extend their operations across north and west Africa.
Although Western security experts remain doubtful about al-Shabaab's tradecraft and ability to carry out sophisticated terrorist attacks far from its operational center, the U.S. global security consultancy Stratfor observed that the Addis Ababa incident underlined the group's determination "to act on threats they have made against the countries with troops in Somalia.
"Three years separated the Kampala attack and the Westgate Mall attack, but only a month passed between the Westgate attack and the explosion in Addis Ababa, indicating that al-Shabaab appears to have increased the tempo of its attacks outside Somalia."