(CNN) -- Count by count, Army Staff Sgt. Robert Bales on Wednesday admitted to gunning down 16 civilians in a 2012 rampage through two villages near his outpost in southern Afghanistan.
Bales pleaded guilty to more than 30 criminal charges, including 16 premeditated murder counts, in a hearing before a military judge. He admitted to slipping away from his outpost in southern Afghanistan and going on a house-to-house killing spree in two nearby villages in March 2012, a massacre that further strained ties between American troops and their Afghan allies.
The move spares the 39-year-old Bales the prospect of a death sentence in the killings. But he was unable to offer the judge, Col. Jeffery Nance, an explanation for his actions.
"I've asked that question a million times since then. There's not a good reason in the world for the horrible things I did," Bales replied, according to Drew Mikkelson of CNN affilliate KING, who was tweeting from the courtroom.
Wednesday's hearing was held at Joint Base Lewis-McChord, near Tacoma, Washington. Both Nance and Lt. Gen. Robert Brown, commander of the Army's I Corps, have approved the plea agreement, said Lt. Col. Gary Dangerfield, a spokesman for the installation.
Bales now faces life in prison, but a jury of four officers and two enlisted personnel will decide whether he will have a chance at parole, Dangerfield said. Nance set sentencing for August 19.
"Sergeant Bales has been waiting for the day that he can accept responsibility for what he has done -- the day that he can give, hopefully, some sense of peace to the people who are the victims of this tragedy, to his own family and to the soldiers who are still serving in Afghanistan," defense attorney Emma Scanlan said after the hearing. "That has been his purpose from the beginning, and that remains what he wants from this."
In addition to the murder counts, Bales pleaded guilty to six counts of attempted murder, seven of assault and the use of illicit steroids and alcohol, but pleaded not guilty to a charge of obstruction of justice.
Bales is a member of the Army's 3rd Stryker Brigade Combat Team, an element of the 2nd Infantry Division. His attorneys have said the service made a mistake in assigning Bales to another combat tour despite evidence of post-traumatic stress disorder and a traumatic brain injury suffered during a combat tour in Iraq.
In the penalty hearings, Scanlan said the defense also will argue that Bales' problems were also fueled by drugs and alcohol provided by Special Operations troops at his outpost before the killings.
"We know all of those things to be true, as does the government," Scanlan told reporters after the hearing. "You take that with somebody on their fourth deployment and the stresses of combat, and we get in some parts of the situation we are in today."
But Scanlan added, "There's no justification" for the massacre. "He doesn't have one, and neither do we."
U.S. and allied troops invaded Afghanistan in 2001 in response to the September 11 al Qaeda attacks on New York and Washington. The invasion quickly routed Afghanstan's ruling Taliban, which had allowed al Qaeda to operate from its territory, but the Taliban regrouped and have mounted a guerrilla war against the allied-backed government of Afghan President Hamid Karzai ever since.
More than 2,200 Americans and nearly 1,100 allied troops have been killed since the war began in October 2001. Nearly 15,000 civilians have died since 2007, the United Nations reported in February.
The United States plans to remove combat troops from the country by the end of 2014.
CNN's Barbara Starr and Devon Sayers contributed to this report.