force to set up observation posts for detection of the enemy, and to permit the concealed movement of troops to the ambush position and the dispersal of troops during withdrawal. One important feature of the ambush was that the target units should 'pile up' after being attacked, thus preventing them any easy means of withdrawal from the kill zone and hindering their use of heavy weapons and supporting airstrikes and artillery fire. Terrain was usually selected which would facilitate this and slow down the enemy. The terrain around the ambush site which was not favorable to the ambushing force, or which offered some protection to the target, was heavily mined and booby trapped or pre-registered for mortars. Iraqi insurgents frequently launched ambushes of Coalition and Iraqi military convoys and patrols during the Iraq War, using numerous types of weapons such as small arms, rocket-propelled grenades, snipers, improvised explosive devices and car bombs. Soft-skinned vehicles such as Humvees were the most commonly targeted. The congested and constricted terrain of the urban areas, and in the rural areas, palm groves and other crops, offered idea cover and concealment for insurgents launching ambushes. The Shia Islamic insurgent group known as the Mahdi Army was one of the primary users of ambush tactics, but ambushes were also used by the Jihadist groups Al-Qaeda in Iraq and Jamaat Ansar al-Sunna. Attacks were usually broken off before support could be called in, in traditional guerrilla fashion. Direct ambushes of U.S. forces declined in the later stages of the war, however, to avoid insurgent casualties as U.S. defences and tactics improved. Ambushes against the poorly equipped and experienced Iraqi security forces, however, proved very lethal. Most Mahdi Army ambushes prior to April 5, 2004 involved no more than seven ambushers with kill zones no larger than 100 meters, however after that night, kill zones became much larger (some several hundred meters long) with many more attackers. There were occasional isolated cases of larger ambushes, such as an attack on a coalition convoy in Samarra on November 30, 2003 that involved 100 fighters and a massive ambush of a coalition convoy in Sadr City on April 4, 2004 by over 1,000 Mahdi Army militiamen. Assassinations Insurgent groups have often employed assassination as a tool to further their causes. Assassinations provide several functions for such groups, namely the removal of specific enemies and as propaganda tools to focus the