The increased number of U.S. soldiers did allow U.S. forces to disarm some Sunnis in Baghdad. Then, the U.S. occupiers invited Shiite militias to invade Sunni neighborhoods and ethnically cleanse them. By mid 2007, Baghdad, once about 65% Sunni, emerged as a predominantly Shiite city. Indeed, leader of the multinational armies in Iraq General David Petraeus, now in charge of Central Command, purposely or inadvertently encouraged Shiites to drive Sunnis from their homes. Many went to Syria. (George Hunsinger, Common Dreams.org, October 23, 2008)
In other words, Bush was paying unknown quantities of U.S. taxpayers' money to Iraqis in return for them not attacking U.S. forces. So, while the infusion of more U.S. troops played some role in cutting down violence, it didn't compete with the part played by death squads. Bob Woodward in The War Within (Simon & Schuster, 2008) suggests that by creating Iraqi "Death Squads" the Pentagon also helped reduce fighting in Iraq. A "Top Secret" memo, according to Woodward, implies that U.S. forces targeted certain Sunni groups for systematic assassination. This operation, like the CIA's Phoenix Program in Vietnam, called for killing those who refused to "reconcile" to U.S. reason; they wouldn't even take bribes.
Thus far, thousands of Iraqi professors, scientists, and doctors have been assassinated. Bush's rescue of Iraq also cost the lives of some 350 journalists. Tens of thousands of prisoners remain in detention camps and, according to a UN report, "the detention of children in adult detention centers violates U.S. obligations under the UN Convention on the Rights of the Child, as well as accepted international human rights norms." (AP, May 19, 2008)