its aftermath. In preparation for the invasion, 100,000 U.S. troops were assembled in Kuwait by 18 February. The United States supplied the majority of the invading forces, but also received support from Kurdish irregulars in Iraqi Kurdistan. The invasion was preceded by an air strike on the Presidential Palace in Baghdad on 19 March 2003. The following day coalition forces launched an incursion into Basra Province from their massing point close to the Iraqi-Kuwaiti border. While the special forces launched an amphibious assault from the Persian Gulf to secure Basra and the surrounding petroleum fields, the main invasion army moved into southern Iraq, occupying the region and engaging in the Battle of Nasiriyah on 23 March. Massive air strikes across the country and against Iraqi command and control threw the defending army into chaos and prevented an effective resistance. On 26 March the 173rd Airborne Brigade was airdropped near the northern city of Kirkuk where they joined forces with Kurdish rebels and fought several actions against the Iraqi army to secure the northern part of the country. The main body of coalition forces continued their drive into the heart of Iraq and met with little resistance. Most of the Iraqi military was quickly defeated and Baghdad was occupied on 9 April. Other operations occurred against pockets of the Iraqi army including the capture and occupation of Kirkuk on 10 April, and the attack and capture of Tikrit on 15 April. Iraqi President Saddam Hussein and the central leadership went into hiding as the coalition forces completed the occupation of the country. On 1 May, an end of major combat operations was declared, ending the invasion stage of the Iraq War and beginning the military occupation period and the Iraqi insurgency against coalition forces. History First phase of insurgency (2003–06) The Iraqi insurgency of 2003–06 erupted following the invasion of Iraq and the toppling of Saddam Hussein's rule in May 2003. The armed insurgent opposition to the United States-led multinational force in Iraq and the post-2003 Iraqi government lasted until early 2006, when it deteriorated into a sectarian civil war, the most violent phase of the Iraq War. Sectarian civil war (2006–2008) Following the U.S.-launched 2003 invasion of Iraq, the situation deteriorated, and by 2007, the intercommunal violence between Iraqi Sunni and Shi'a factions was described by the National Intelligence Estimate as having elements of a civil war. In a 10