The military’s highest court overturned a murder conviction Wednesday against a Marine from Plymouth, Mass., in one of the most significant cases against American troops from the Iraq war. The Court of Appeals for the Armed Forces threw out the conviction of Sergeant Lawrence Hutchins III, who has served about half of his 11-year sentence. According to the ruling posted on the court’s website, the judges agreed with Hutchins, who said his constitutional rights were violated when he was held in solitary confinement without access to a lawyer for seven days during his 2006 interrogation in Iraq. The decision is seen as a major blow to the military’s prosecution of Iraqi war crimes. Hutchins led an eight-man squad accused of kidnapping an Iraqi man from his home in April 2006, marching him to a ditch, and shooting him to death in the village of Hamdania. Hutchins has said he thought the man, who turned out to be a retired policeman, was an insurgent leader. The Navy can appeal to the Supreme Court or send the case to the convening authority, who can either order a retrial or let the ruling stand.