It began when military forces from Ethiopia, supported by the United States, invaded Somalia to depose the Islamic Courts Union (ICU) and install the Transitional Federal Government (TFG).

In early 2007 an insurgency began, centered on a loose coalition of Islamic Courts loyalists, volunteers, clan militias, and various Islamist factions, of which Al-Shabaab eventually assumed a pivotal role.

[53] Yusuf was previously a member of an Ethiopian-backed coalition of warlords that had undermined the TNG,[54] and decades prior to that had led Somali rebels who assisted invading Ethiopian troops during the 1982 Ethiopian–Somali war.

[69] British television station Channel 4 acquired a leaked document detailing a confidential meeting between senior American and Ethiopian officials in Addis Ababa six months prior to the full scale December 2006 invasion.

The Economist reported that the Ethiopian military incursion had set off a fierce reaction even among the most moderate of the ICU, and a recruitment mobilization began in order to raise a force to take back Burhakaba.

[94] The Ethiopian military campaign against the ONLF, along with widespread atrocities committed against civilians associated with it, drove hundreds of men (thousands according to some estimates) from the Ogaden to Mogadishu in order to answer the ICU's call to arms against the invasion.

[95] Several hundred men from Somaliland also joined the Courts militia, including high-ranking military officers, while ICU supporters in Puntland primarily provided financial and logistical aid.

The resolution was widely viewed by the Courts as the UN Security Council unjustly legitimizing an Ethiopian invasion, considering the UNSCR had refused to make any commentary or statement on the troops already deployed inside of Somalia.

Approximately 50,000 to 60,000 Ethiopian National Defence Force (ENDF) troops backed by tanks, helicopter gunships and jets had been involved in the offensive against the Islamic Courts Union during December 2006.

[115] The insurgency that followed the collapse of the ICU was composed of numerous different groups and factions, making it difficult to determine who was responsibility for a variety of attacks and incidents, though Al-Shabaab ultimately became the most powerful and active element.

[59][102] On 13 December 2006, two high-ranking officials in the ICU's military wing, Yusuf Indhacade and his deputy Mukthar Robow, gave Ethiopian troops deployed in Somalia a seven-day ultimatum to withdraw from the country or face expulsion.

[102] The Courts were divided over whether or not to forcibly eject invading Ethiopian troops, and the European Union began last minute diplomatic efforts to halt the outbreak of war, resulting in contradictory statements from various ICU leaders.

Sharif Sheikh Ahmed and Hassan Dahir Aweys, both adopted conciliatory stances as a result, but the sharp rise in tensions had empowered the Courts 'Hawks' who viewed the growing Ethiopian military forces and the passing of UNSCR 1725 as proof of an international conspiracy against the ICU.

[124] ICU fighters, many of whom were university students, attacked Ethiopian positions in Daynunay, 20 kilometres east of Baidoa as heavy fighting and artillery shelling broke out on several different front lines.

[130] US intelligence sources reported that in the initial days of the conflict, the ICU effectively utilized tactics against ENDF tanks that mirrored those employed by Hezbollah against the IDF months prior during the Lebanon War.

[129] In an interview with Al-Jazeera, head of the Islamic Courts Sharif Sheikh Ahmed later reported that after achieving a string of battlefield victories, ICU troops had come under unexpected bombardment from US aircraft.

According to David Shinn, US ambassador to Ethiopia, the ICU had recognized their vulnerability to sustained attacks from Ethiopian air and armor superiority in conventional warfare and had opted for a transition to insurgent tactics.

[154] The ICU had evacuated many towns without fighting as ENDF/TFG forces advanced on Mogadishu[155] That same day the African Union, supported by the Arab League and the IGAD, called for Ethiopia to withdraw from Somalia immediately.

[159] At the start of January, the Ethiopian government claimed it would withdraw "within a few weeks"[170] The TFG announced that the rivaling Islamic forces had been defeated and that no further major fighting was expected to take place.

ICU insurgents, Hawiye clan militia, volunteers and other Islamist groups engaged in fierce rounds of fighting in dense urban eras for several weeks during March and April against ENDF/TFG forces.

[71] Human Rights Watch reported that the Ethiopian army extensively utilized BM-21 Grad rocket shelling to bombard densely populated Mogadishu neighborhoods, which the organization described as a violation of international humanitarian law.

[39] According to Kenyan journalist Salim Lone, ENDF and TFG forces deliberately blockaded the delivery of urgently needed humanitarian supplies and food in an attempt to 'terrify and intimidate' civilians associated with those challenging the military occupation.

[211] As fighting in Mogadishu escalated, ICU insurgents in southern Somalia found a window of opportunity and peacefully captured the town of Dhobley near the Kenyan border in mid October.

[221] According to Professor Abdi Ismail Samatar, as the insurgency grew in strength, it became clear to the Americans that the Ethiopian military occupation was doomed to fail, prompting them to focus on engineering a split within the Islamic resistance movement.

[71] The invasion resulted in the deaths of many Islamic Courts Union affiliates, leaving a vacuum for the small group of several hundred youth that served as the ICU's Shabaab militia to gain prominence.

Heavy handed tactics and blatant disregard for civilian life by Ethiopian troops rallied many Somalis to support the al-Shabaab as it successfully branded itself as the most determined and uncompromising resistance faction.

The Swiss newspaper Le Temps reported that the large volume of Ethiopian military arms and ammunition being sold suggested high-level involvement within the ENDF as commanders often resold weapons seized from insurgents for profit.

[258] The ENDF shelled the western part of Beledweyne with rocket and mortar fire, resulting in an exodus of civilians[259] and the city saw fierce fighting between Courts fighters and the Ethiopian army in the following weeks.

[282] Ethiopian president Meles Zenawi declared the mission had been a success, but the operation had had proved to be effectively futile as the transitional government Ethiopia had backed during the war found itself completely powerless in the lead up to the ENDF withdrawal.

[33] Under the command of Colonel Gabre Heard, nicknamed 'Butcher of Mogadishu', soldiers from the Tigrayan Peoples Liberation Front (TPLF) participating in the occupation routinely bombed civilian areas and killed thousands.