There were fears that al-Shabaab would unleash a wave of terror against Somalia's Western-backed neighbors in East Africa who have hammered the Islamists over the last year.
"It will become a kind of asymmetric warfare, with more suicide bombings and attacks targeting whoever eventually controls Kismayo," warned Horn of Africa analyst Andrews Atta-Asamoah of South Africa's Institute for Security Studies.
The Islamists abandoned Kismayo, 300 miles south of Mogadishu, as the long-awaited Kenyan assault got under way with attacks on three sides, spearheaded by a pre-dawn amphibious assault.
Revenge came swiftly. Kenyan authorities said two police officers were killed Sunday in the northern town of Garissa hours after a child was slain in a grenade attack on a church in Nairobi, Kenya's capital.
Security authorities said they feared the attacks on Kenya, the region's biggest economy, were precursors of a wider onslaught of revenge as al-Shabaab apparently dispersed into the countryside, presumably to wage a hit-and-run guerrilla war again the Western-backed Transitional Federal Government.
Kenyan newspapers reported that Nairobi police found bomb-making equipment aboard a bus from Garissa, 110 miles west of the Somali border, Friday.
In July, masked raiders mounted simultaneous attacks with gunfire and grenades on two churches in Garissa, killing 17 people.
Kenya is largely Christian but some young Muslims are known to be fighting alongside al-Shabaab.
Kenya, with U.S. support, launched its offensive eastward into Somalia against al-Shabaab in October 2011, as an Ethiopian column struck southward against Kismayo, which the Islamists had held since 2007 and has been a key revenue base for them.
At the same time, Ugandan and Burundian troops of the 17,000-person African Union peacekeeping force, supporting Somalia's shaky U.S.-backed Transitional Federal Government, mounted an offensive from the west that drove al-Shabaab out of Mogadishu.
Several key towns held by al-Shabaab have fallen in recent months as the three-pronged assault moved forward, supported by naval gunfire from the Kenyan navy, and air strikes by the Kenyan air force.
These were aided by unacknowledged U.S. satellite intelligence and drone strikes mounted from Ethiopia and Djibouti, a former French colony on the Horn of Africa where a U.S. counter-terrorism force has been based since the Sept. 11, 2001, attacks on the United States.
Al-Shabaab, which formally joined forces with al-Qaida in February, has also retaliated against Uganda. In July 2010, 79 people were killed in two suicide bombings in Kampala.
The Islamists' attacks on Somalia's neighbors, initially seen as isolated outrages, are being viewed more seriously. Attacks on tourists, a key source of revenue, in 2011 triggered the Kenyan push into Somalia.
The stakes have gone up in recent months amid major oil and natural gas strikes in Kenya, Uganda, Tanzania and Mozambique.
U.S., Chinese, Indian and European interests in this bonanza, along with others in West Africa, have changed the geopolitical complexion of the current conflict in Somalia, that began when U.S.-backed Ethiopian armored forces drove out a short-lived Islamic regime in December 2006.
Al-Shabaab -- which includes foreign, mainly Arab, veterans of al-Qaida's wars -- launched a rebellion when the TFG was installed in Mogadishu and at one time controlled about 80 percent of Somalia.
The TFG was seen as inept and corrupt, riven by the clan rivalries that plunged Somalia into anarchy after the dictator Mohammed Siad Barre was toppled in 1991.
The fall of Kismayo, a city of 200,000, followed the Sept. 10 election of a new president, Hassan Sheikh Mohamud, a university professor seen as a clean-hands reformer.
His inauguration raised hopes that Somalia may be moving into a new era. But that optimism was shattered by a series of al-Shabaab suicide bombings in Mogadishu, including one that targeted the new president Sept. 12.
"Perhaps the biggest threat is the vacuum al-Shabaab leaves behind," observed British analyst David Smith.
"While ... Mohamud has more legitimacy than his predecessors, the clan factionalism that has tormented Somalia for so long has not gone away ... A generation of conflict is likely to take a generation to heal."