Pfc. Jesse Spielman pleaded to conspiracy to obstruct justice, arson, wrongfully touching a corpse and drinking.
He still faces trial on the more serious charges in the March 2006 attack on Abeer Qassim al-Janabi and her family. Under military law, a soldier present when a crime occurs can be found guilty if prosecutors can establish that the soldier had prior knowledge.
Three other soldiers have pleaded guilty for their roles in the crimes and received sentences as long as 100 years. Another soldier was discharged from the military before he was charged and could face the death penalty if found guilty in federal court in Kentucky.
Defense attorney Craig Carlson said Spielman's plea to the lesser charges was part of an agreement with prosecutors that involved crimes that Spielman had already confessed to committing during interviews with military investigators.
A military judge later in the day began seating a jury for his court-martial on the rape and murder charges.
Defense attorneys have argued that Spielman had no prior knowledge of the attack. On Monday, they requested immunity for members of the Army's combat stress team, which was attached to Spielman's unit.
Carlson argued that questionable practices and undocumented distribution of medication to soldiers could have left them in a mental state in which they were unable to recognize the nature of the crime. The Department of Justice declined to grant immunity in June, but defense attorneys say the information is critical to their arguments.
The defense also filed a motion to exclude nine photos taken of the victims the day after they were killed.
Col. Stephen Henley, the military judge, said he would allow all but one of the photographs taken by soldiers who responded to the home after hearing reports from Iraqi police the family had been slain.
"While the photographs are bloody, they appear to accurately reflect the crime scene," Henley said.
Prosecutors had argued the photos provide crucial information about the crime.
"These photos painfully demonstrate that she is dead," said Maj. William Fischbach of photos of the 14-year-old girl. He said the position of her body in the photos suggested she had been raped.
"Taken together, these crime scene photos are the government's only evidence that a murder happened," Fischbach said.
Two soldiers have told investigators that Spielman, 22, of Chambersburg, Pa., knew of the plan to rape the girl in Mahmoudiya, a village 20 miles south of Baghdad, and was present when they set the details over swigs of whiskey.
During their courts-martial, Spc. James P. Barker and Sgt. Paul E. Cortez testified they took turns raping the girl while then-Pfc. Steven D. Green shot and killed her mother, father and younger sister. They said Green, who is accused of being the ringleader, shot Abeer in the head after raping her. The girl's body was then set on fire with kerosene to destroy the evidence, according to testimony and military documents.
Green has pleaded not guilty in federal court to charges including murder and sexual assault. No trial date has been set.
Barker said Spielman came to the home knowing of the plan. Pfc. Bryan L. Howard, who stayed at the checkpoint to monitor radios, testified during a hearing in March that he overheard Spielman and the others discuss the rape beforehand. Howard pleaded guilty to being an accessory to the rape and murder and was sentenced to 27 months.
Cortez and Barker pleaded guilty to rape and murder; Barker was sentenced to 90 years and Cortez to 100 years.