Shiite regions of Iraq. On November 8, the assault on Fallujah began in Operation Phantom Fury. Many guerrillas had apparently slipped out amongst the fleeing civilians, leaving a force of 5,000 still remaining in the city out of a force of what had grown to 10,000 insurgents (according to estimates). The vast majority of the civilian population by this time had fled, although thousands remained (either because they lacked means of transportation or because they were guarding their property). A concentrated barrage of air strikes and artillery (including use of white phosphorus and napalm) began pounding the city. Simultaneously, U.S. and Iraqi forces began advancing, primarily from the north, and quickly secured the city's main hospital, where rebels had reported high numbers of civilian deaths in April. Guerrilla resistance was initially reported to be less than expected, partially due to diversionary tactics and also because much of the insurgent force in Fallujah had fled. Ruined homes across the city attest to a strategy of overwhelming force. A video showing the killing of at least one unarmed and wounded man by an American serviceman surfaced, throwing renewed doubt and outrage at the efficiency of the US occupation. The Marine was later cleared of any wrongdoing, as the Marines had been warned that the enemy would sometimes feign death and booby-trap bodies as a tactic to lure Marines to their deaths. November was the deadliest month of the occupation for coalition troops, surpassing April. By November 9, U.S. units had penetrated into the heart of the city. By now, they were receiving stiff opposition from small groups of guerrillas, employing hit-and-run tactics, and snipers. Booby traps, rigged to destroy homes U.S. troops had entered, also were encountered, including some even attached to corpses. Within a week, 38+ U.S. troops had been killed and 275+ wounded in the offensive. Insurgent losses during that same time period were believed to be much heavier, running into several hundred. Gradually, U.S. and Iraqi forces pushed the main insurgent force into the southwest of the city, carefully conducting house-to-house searches and securing areas of the city block by block. The U.S. military adopted a "hammer and anvil" strategy, in which they hoped to methodically push the insurgents into the southwestern corner of the city and up against the open desert (where they would be without cover or concealment and open to a final rout by U.S. firepower).