Ian Fishback, an Army whistle-blower whose allegations that fellow members of the 82nd Airborne Division in Iraq routinely beat and abused prisoners prompted the Senate to approve anti-torture legislation in 2005, died on Nov. 19 in Bangor, Mich. He was 42.

His family, which announced the death in a statement, said the cause had not been determined. In the climax to a distinguished but abbreviated career that the family said had begun to unravel as a result of neurological damage or post-traumatic stress disorder resulting from combat tours in Iraq and Afghanistan, he died in an adult foster care facility where he had been admitted following court-ordered treatment with anti-psychotic drugs after he became delusional and created public disturbances.

Major Fishback was one of three former members of the division who said soldiers in their battalion had systematically abused prisoners by assaulting them, exposing them to extreme temperatures, stacking them in human pyramids and depriving them of sleep to compel them to reveal intelligence — or, in some cases, simply for the American soldiers’ amusement. He said his complaints were ignored by his superiors for 17 months.

He reported some of the abuses in September 2005 in a letter to top aides of two senior Republicans on the Senate Armed Services Committee: John W. Warner of Virginia, the chairman, and John McCain of Arizona. The aides said his reports were sufficiently credible to warrant investigation.

Additional allegations from two other members of the division were included in a report released later that month by Human Rights Watch.

“Ian’s greatest quality is not his courage, but his humanity,” Christopher Nicholson, a former Army buddy, wrote on gofundme.com, where by the time of Major Fishback’s death friends had raised more than $18,000 toward a goal of $60,000 to transfer him to the Austin Riggs Center, a private psychiatric treatment facility in Stockbridge, Mass.

“I always marveled at the way he could shoot at and be shot at by terrorists, watching his friends die in battle, then in the very next instant risk himself to demand that the prisoners be treated with decency,” Mr. Nicholson wrote. “I remember I once called him an expert on warfare and he looked mildly offended and responded that he was an expert on justice.”

In his letter to the senators, Major Fishback said that troops were often torn among what they were trained to do, instructions in field manuals, orders from superiors and the exigencies of actual combat.

“I am certain that this confusion contributed to a wide range of abuses including death threats, beatings, broken bones, murder, exposure to elements, extreme forced physical exertion, hostage-taking, stripping, sleep deprivation and degrading treatment,” he wrote. “I and troops under my command witnessed some of these abuses in both Afghanistan and Iraq.”

“Do we sacrifice our ideals in order to preserve security?” he continued. “Will we confront danger and adversity in order to preserve our ideals, or will our courage and commitment to individual rights wither at the prospect of sacrifice?”

He concluded his letter: “I strongly urge you to do justice to your men and women in uniform. Give them clear standards of conduct that reflect the ideals they risk their lives for.”

Later that year, the Senate voted 90 to 9 to approve Senator McCain’s Detainee Treatment Act, which prohibited “cruel, inhuman or degrading treatment or punishment,” although subsequent amendments carved out caveats.

Time magazine named Major Fishback one of the 100 most influential people in the world that year.

Ian Fishback was born on Jan. 19, 1979, in Detroit. His parents, John and Sharon Fishback, were both rural letter carriers.

He grew up in Newberry, a village of about 1,500 on Michigan’s Upper Peninsula that bills itself as the state’s “official moose capital.” In 1997 he graduated from Newberry High School, where he excelled in football and wrestling and achieved a 3.953 grade point average (out of 4) and where, his father said, he decided to pursue a military career.

“He was looking for a way to do better in the world,” said Justin Ford, a boyhood friend who organized the funding drive. “He was looking for structure.”

He graduated from the United States Military Academy at West Point with a Bachelor of Science degree in Middle Eastern studies in 2001. He served in the Army until 2014, including two combat tours with the 82nd Airborne and two with the Fifth Special Forces Group.

“He had all the scars from it,” Brig. Gen. Stephen N. Xenakis, a retired medic who had worked with Major Fishback since 2005 on human rights issues, said in an interview.

“It was not that he was a perfectionist,” General Xenakis added. “I think he wrestled with understanding what are the principles, what am I supposed to do, how am I supposed to organize my conduct and thinking? He was intent on doing what he thought was ethical.”

Major Fishback earned a master’s degree in philosophy and political science at the University of Michigan in 2012, taught at West Point from 2012 to 2015, and was awarded his doctorate from the University of Michigan. In his thesis, dated this year, he explored the questions of when a war is just, when a soldier has a moral justification to disobey orders, and what the scope of his responsibility is both for doing harm and for allowing harm to be done.

His marriage to Clara Hoisington, a fellow West Point graduate, ended in divorce. He is survived by their young daughter; his parents, John Fishback and Sharon Ableson; his stepmother, Sharon Brown; and his sister, Jazcinda Jorgensen.

Mr. Ford, his friend from boyhood, described Major Fishback as a “moral absolutist.”

“If I asked him to help me bury a body, he would turn me in,” Mr. Ford said. “He would have been a great moral compass for this country.”

Major Fishback said several years ago that his original testimony on abuses had been discredited by the Army, in part because doctors said he was suffering from post-traumatic stress disorder.

Although he was promoted to major from captain, Major Fishback decided to leave the Army and the United States altogether. He moved to Sweden to accept a Fulbright scholarship, worked for a human rights organization, applied for European Union citizenship and sought, he said, to “make sure Europe is able to fend off the United States and Russia.”

“I’m done,” he told Carol Stiffler, the editor of the weekly Newberry News and a former classmate of his sister, in January 2020. “I gave the U.S. a lifetime of service — very admirable service. And if this is the repayment, it is not acceptable.”

At the time, his father called him “a natural-born warrior” who “was simply standing up for the rule of law.”

Major Fishback’s departure was delayed by the pandemic, though, and he returned home from Sweden after his life had begun to fall apart.

He began receiving psychotropic drugs and was involuntarily committed in September, when his behavior became erratic, resulting in an arrest at a football game. His father said that as of last month he was still depressed, but that he was “ditching his demons” and “coming back to reality.”

“We know the community supported Ian through his recent difficult times,” the Fishback family said in its statement. “He faced many challenges, and many of us felt helpless. We tried to get him the help he needed. It appears the system failed him utterly and tragically.”

“We will seek justice for Ian,” the statement concluded, “because justice is what mattered most to him.”