Origin of the conflict
Rebel forces ousted the repressive regime of Siad Barre in 1991. A counter-revolution took place to attempt to reinstate him as leader of the country. Since dictator Barre fled the country in 1991, Somalia has been without a stable central government.

Chronological timeframe
1993:
UN humanitarian troops started a two-year effort to alleviate famine conditions.
Several gun battles in the capital (Mogadishu) between local gunmen and peacekeepers have caused the death of 24 Pakistanis and 19 US soldiers (this incident is the basis for the book and movie Black Hawk Down).
1995:
The UN withdrew after suffering more significant casualties. Order in Somalia still had not been restored.
1998–2006:
Declaration of a number of self-declared autonomous states within Somalia. Movements such as the interclan Transitional National Government and the Somalia Reconciliation and Restoration Council eventually led to the foundation of the Transitional Federal Government (TFG). However, warlord and clan-based violence continued government movements had little control over the country.
2006:
The Supreme Islamic Courts Union (ICU) captured the capital and they expanded to the borders of Puntland and took over southern and middle Jubaland. The Islamic movement's growing power base and militancy led to increasingly open warfare between the Islamists and the other factions of Somalia. Ethiopia decided to intervene, who supported the secular forces of Somalia.
The ICU and TFG began the Battle of Baidoa. The ICU aimed to force the Ethiopians off Somali soil. However, they were defeated in all major battles and forced to withdraw to Mogadishu. After the brief final action at the Battle of Jowhar, the leaders of the ICU resigned.
2007:
Prime Minister Ali Mohammed Ghedi called for the country to begin disarming.
The United States officially militarily interceded in the country for the first time since the UN deployment of the 1990s.
The troops of ICU has began a guerrilla guerrilla warfare against Ethiopian and Somali government forces and existing tribal conflicts continued.
2009:
Ethiopian soldiers withdrew from Somalia. Several thousand of the African Union stayed to help the fragile coalition government and its troops enforce their authority.
A power sharing agreement between Islamists splinter group Alliance for the Re-liberation of Somalia and the TFG has come into force.
The southern half of the country rapidly fell into the hands of radical Islamist rebels after Ethiopoa left. The rebels quickly routed the government and African Union troops in several provinces, establishing sharia law in areas under their control.

Involved actors
Insurgents: want control over the country, because they wish to install the sharia law. The main fighters are from al-Shabab, a radical faction that emerged from the ICU.
Somali Transitional Federal Government: the interim Parliament of Somalia is formed in 2004. The four major clans are represented in the government.
Ethiopia: intervened because it faced a direct threat to its own borders. It wants to save the interim government and prevent fundamentalist Islam to be spread in Africa.

Reaction of the international community
Ethiopia intervened in Somalia because as a neighbouring country it faced a direct threat. However, it suffered daily attacks and Ethiopian soldiers died. Ethiopia fears, just like the US, the spread of fundamentalist Islam.
The United Nations backed the peace process which led to the election of Sheikh Sharif Sheikh Ahmed as president. The UN has sent humanitarian troops in the beginning, but it withdrew these after suffering several casualties.
In April 2009, the international community pledged more than $200m to Somalia to increase security in the country. Most of the money is for the African Union to help them with equipment and the training of Somali forces.

Sources
http://www.globalsecurity.org/military/world/war/somalia.htm
http://en.wikipedia.org/wiki/Somali_Civil_War
http://news.bbc.co.uk/2/hi/africa/4760775.stm