The US has intervened militarily in Somalia for the first time since its disastrous mission in the 1990s.
The US set up an anti-terror base close to Somalia after 9/11
It is believed to have launched an air strike against Islamist fighters - possibly including suspected al-Qaeda operatives hiding in southern Somalia - following the defeat of the Islamist group which had taken control of much of the country last year.
Washington is determined to prevent the spread of fundamentalist Islam to Africa and was deeply concerned by the rise of the Union of Islamic Courts in Somalia.
One of the UIC leaders, Sheikh Hassan Dahir Aweys, is on a US list of those with links to terror groups - he denies the charges.
The US has seen the fight against terrorism as its highest priority in Africa ever since 7 August 1998, when two car bombs exploded outside the American embassies in the Kenyan capital, Nairobi, and Dar es Salaam in Tanzania.
Al-Qaeda claimed responsibility for the explosions, which killed more than 250 people and left 4,000 wounded.
The US has named Comoros national Fazul Abdullah Mohammed, Kenyan Saleh Ali Saleh Nabhan and Abu Taha al-Sudani of Sudan as suspects in the bombings it believes to have been sheltering in Somalia.
Osama Bin Laden has also claimed that his fighters were involved in the killing of 18 US troops in Mogadishu in 1993, when a US Black Hawk helicopter was shot down.
This prompted their humiliating withdrawal from the country the following year.
Following the 9/11 attacks on the US in 2001, a US military base was set up in Somalia's neighbour, Djibouti, to form the Combined Joint Task Force - Horn of Africa.
The mission of its 1,500 troops is described as "detecting, disrupting and ultimately defeating transnational terrorist groups operating in the region. Denying safe havens, external support and material assistance for terrorist activity."
From Djibouti it has launched a number of operations: working with the Ethiopians in anti-terrorism activities in Ethiopia's Somali-speaking region of the Ogaden.
The US is believed to have backed Ethiopia and government troops against Somalia's Islamists
They have monitored the boats plying their trade between the Arabian peninsular and Africa.
The Task Force is part of US Central Command, which has mounted special operations inside Somalia, to fight groups labelled as terrorists.
Little is ever said about all of this work, but it is believed that several terror suspects have been seized in Mogadishu and flown abroad by the CIA.
It is also widely assumed that the US has been supplying intelligence to the Ethiopian and Somali government forces who have driven the Islamic courts' militia from Mogadishu in recent weeks.
With the deployment of warships, including the aircraft carrier Dwight D Eisenhower off the Somali coast, BBC Africa analyst Martin Plaut says the American operation has come out of the shadows and into the glare of publicity.