SAN DIEGO – In the biggest prosecution of U.S. troops in the Iraq war, the highest-ranking serviceman to face court-martial involving combat since Vietnam was ordered to trial for failing to investigate the killings of 24 Iraqis in Haditha two years ago. Another Marine was also ordered Friday to face court-martial for charges including involuntary manslaughter in relation to the killings, which included women and children. Lt. Col. Jeffrey R. Chessani faces charges of dereliction of duty and violation of a lawful order on allegations that he mishandled the aftermath of the Nov. 19, 2005, shootings, which followed a roadside bombing that killed a Marine driver. Chessani was commander of the Camp Pendleton-based 3rd Battalion, 1st Marine Regiment that has been the focus of the prosecution. “I can tell you this decision by Gen. Mattis today is going to have a negative affect on all officers, including battalion commanders,” he said, adding it would undermine the trust between commanders and their troops. “Are they going to be able to trust the word of their junior officers, senior enlisted and junior enlisted?” If convicted on all counts, Chessani faces up to three years in prison.160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! He is the most senior U.S. serviceman since the Vietnam War to face a court-martial for actions or decisions made in combat, said Gary Solis, a former Marine Corps prosecutor and judge who teaches law of war at Georgetown University Law Center. One of Chessani’s men, Lance Cpl. Stephen B. Tatum, was ordered to face a court-martial on charges of involuntary manslaughter, reckless endangerment and aggravated assault. He is one of four Marines originally charged with murder in the killings. The decision by Lt. Gen. James Mattis to send Tatum to court-martial comes after the investigating officer said last month that the evidence was too weak to prosecute him. But Tatum will not be tried on the murder count he originally faced. Tatum, of Edmond, Okla., shot and killed civilians, but “he did so because of his training and the circumstances he was placed in, not to exact revenge and commit murder,” Lt. Col. Paul Ware wrote last month in recommending he not face court-martial. Chessani’s civilian defense attorney, Brian Rooney, told The Associated Press he was disappointed with the general’s recommendation.