RAF FAIRFORD, England — An upset Airman 1st Class Mark Cooper just wanted to keep his infant son from moving so he could change his diaper following a dispute with his wife, who said he was mishandling their baby.

To make his 6-week-old son stay still, the airman put pressure on his son’s legs with his hands until suddenly he heard something break and the baby begin to cry louder, he testified in court.

On Wednesday, a military judge, Lt. Col. Jennifer Cline, found the 420th Munitions Squadron airman guilty of assaulting his son in the March 17, 2007, incident.

Cline sentenced Cooper to 18 months in jail, reduced him in rank to E-1, and gave him a bad-conduct discharge.

“The excessive force he used caused one of the largest bones in the body to snap,” prosecutor Capt. Etienne Miszczak said during Cooper’s court-martial.

Once the infant was admitted to the Great Western Hospital in Swindon, it was found the left femur bone had shattered in half. X-rays also showed six other bone fractures in his legs and both collarbones.

In the three-day trial, prosecutors relied on the testimony of two British medical experts.

Dr. Helen Price, a consultant pediatrician at the Swindon hospital, testified that she ran various tests to cancel out bone-related disorders before concluding that the baby suffered “a series of non-accidental injuries.”

Dr. Stephen Chapman, a pediatric radiologist at the Birmingham Children’s Hospital, testified to the seven different fractures in the baby’s X-rays.

In closing, defense attorney Capt. Lance Aiumopas argued that there were missing facts in the case and that prosecutors were asking the judge to guess if Cooper’s actions resulted in every fractured bone on his son.

“You can’t rule out other people in these injuries,” Aiumopas told the court Wednesday.

But before his sentencing, Cooper gave an emotional plea to the judge.

“I have no excuse for what I’ve done,” said Cooper, who stopped briefly in his remarks to wipe his tears away. “I can never forgive myself for what I’ve done to my son. I hope he can forgive me someday.”

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