Lt. Col. William H. Steele, 52, had faced a life sentence if convicted of accusations he allowed high-ranking former regime prisoners use of his cell phone for unmonitored calls.
The judge, Lt. Col. Timothy Grammel, found him not guilty of that charge but convicted him of unauthorized possession of classified documents, behavior unbecoming an officer for an inappropriate relationship with an interpreter, and failing to obey an order.
Witnesses were then called to testify before a sentence was to be announced.
The prosecution had argued that Steele had a history of flouting the rules and claimed he loaned an al Qaeda-linked inmate an unmonitored cell phone, despite rules that inmate calls should be arranged in advance and conducted with an interpreter present.
"You heard in this courtroom, in a closed session, that he handed detainee number 2184, an al Qaeda member in Iraq, his personal cell phone and allowed a five-minute conversation. It was the equivalent of putting an AK-47 in his hands," prosecutor Capt. Michael Rizzotti said.
"All it takes is a phone call, and if that detainee can communicate with someone outside, that can put soldiers of the United States at risk," Rizzotti said. "The second he handed over that phone for an unmonitored phone call, in Arabic, that is the second he aided the enemy."
Defense attorney Maj. David Barrett denied that Steele ever provided a cell phone for an unmonitored conversation and said his client was doing his job by treating the detainees in a humane fashion.
"Long after we leave Iraq, and we will leave it, what will be left? It's the impression of the soldiers that will really matter," Barrett said. "Lt. Col. Steele treated the detainees with dignity and respect. Let's not confuse that with sympathy for the enemy."
Barrett also said Steele's storage of classified documents was an "honest mistake" and he argued that the defendant's relationship with an interpreter did not constitute behavior unbecoming an officer.
Steele, an Army reservist from Prince George, Va., chose not to testify in his own defense in the first court-martial on charges of aiding the enemy since the U.S.-led invasion of Iraq in March 2003.
Much of the trial was held behind closed doors when officials said classified information was discussed.
The only other U.S. officer known to have been accused of collaborating with the enemy since the 2003 start of the war was Capt. James J. Yee, a Muslim chaplain who was linked to a possible espionage ring at the Guantanamo Bay, Cuba, military prison. He was eventually cleared and given an honorable discharge.
Steele has already pleaded guilty to three other charges - including wrongfully storing and improperly handling classified information and possession of pornographic videos - which carry a maximum sentence of six years in prison, forfeiture of pay and dismissal from the Army.
He also could face additional maximum sentences of 10 years for unauthorized possession of classified information, six months for failing to obey an order and possibly a year for conduct unbecoming an officer, officials said.
In other developments: