An Army staff sergeant charged with shooting at unarmed Afghans this year pleaded guilty Wednesday in a deal that requires him to testify against other defendants implicated in an alleged scheme to kill civilians for kicks.

Staff Sgt. Robert Stevens, of Portland, Ore., admitted to aggravated assault, lying to investigators and other charges at his court-martial. Prosecutors dropped a conspiracy charge as part of the plea agreement.

A military judge, Lt. Col. Kwasi Hawks, accepted the agreement and late Wednesday sentenced Stevens to nine months in confinement and a reduction in rank to private. Army spokeswoman Maj. Kathy Turner said Stevens must forfeit all pay and allowances during his confinement.

Five soldiers are charged with killing three civilians in Kandahar Province this year.

Stevens is not one of them but acknowledged that in March he followed an order from Staff Sgt. Calvin Gibbs to shoot at two Afghans in a field who posed no threat. Those Afghans were not injured.

Stevens and six others face other misconduct allegations, ranging from drug use to beating up a fellow soldier, stemming from an investigation of the unit's activities.

Stevens could provide valuable testimony against Gibbs, the highest ranking soldier charged in the killings. Gibbs is accused of leading the alleged murder plot and putting together a "kill team" to carry out unprovoked attacks. He insists all the shootings were justified.

Stevens, a friend of Gibbs, told investigators that he joined Gibbs on patrols even though he was in a different unit. In March, Gibbs ordered Stevens and several other soldiers to join him in shooting at two unarmed men standing in a field, Stevens told investigators.

They missed.

"When SSG Gibbs called for us to fire I knew there was not a threat, and that there was no reason to shoot these guys," Stevens said in a written statement.

"I was extremely thankful to find out that we had not killed or wounded either of those two individuals, and I regret not trying to stop Staff Sgt. Gibbs from trying to kill innocent people," he said in a sworn statement.

Stevens also said Gibbs had shown him a finger he claimed to have cut from the body of an Afghan National Army or Afghan National Police member killed by a roadside bomb, and that Gibbs illicitly collected weapons. Others claimed that Gibbs dropped such weapons near the bodies of civilians to make them appear to have been combatants.

During his court-martial Wednesday, Stevens said Gibbs gave him one illicitly obtained grenade. The News Tribune of Tacoma reported that Stevens said he threw it out of a Stryker vehicle and claimed it had been tossed by an enemy combatant — an incident that resulted in him being awarded a combat action badge.

"There was no reason for it," Stevens said in court.

The charges to which he pleaded guilty were aggravated assault with a dangerous weapon, endangering other soldiers, dereliction of duty and making a false statement.

The plea agreement requires Stevens to testify against the other defendants as needed by prosecutors.