President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Nov. 15 issued pardons to three American servicemen who had been implicated, and in one case convicted, in the commission of war crimes. While the president undoubtedly has the legal authority to grant pardons, we believe his decision to exercise that authority in these cases was a grave error.

Although we come from different disciplines — one of us was a general officer who commanded U.S. combat troops in Iraq and Afghanistan and the other was a career prosecutor and diplomat — we share a common interest in the preservation of the military justice system, the rules that serve as its foundation, and the role these two concepts have long played in making the U.S. military the most effective fighting force in world history. Through these pardons, and his earlier unprecedented interventions as these cases were making their way through the justice system, the president has undermined principles that long have served as a bedrock for our armed forces.

In 1863, at the height of the Civil War, President Lincoln asked Franz Lieber, a law professor and former Prussian military officer who had immigrated to the United States, to draft what would become the first comprehensive code of conduct for any country’s military forces during a time of war. Lieber himself had much at stake in how soldiers were treated by opposing forces. He had two sons who fought for the Union and one who fought for the Confederacy; one of them died and another was gravely wounded in the conflict.

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The Lieber Code ushered in the idea that although war would always involve death and destruction, efforts could be made to make it more humane, particularly for those most vulnerable in a time of conflict: civilians, the wounded, and prisoners of war. In the following decades, the United States took a leading role in pressing for more robust international agreement on the rules governing the conduct of war — what became the Hague and Geneva Conventions. Throughout this period, perhaps the strongest proponent for adoption of these rules was the U.S. military, which recognized the value of putting in place laws that would afford protections to our troops when, inevitably, they found themselves in vulnerable situations.

Since President Lincoln issued what was officially called the Code for Government of Armies in the Field, United States military forces have adhered to a strict set of rules and laws that regulate their conduct and, in large part, ensure that our troops are among the most disciplined in the world. Early in their careers, servicemen and women of all branches of the military are instructed in the laws and rules of war. They know that it is both wrong and illegal to engage in acts that long have defined other less scrupulous armies — rape, pillage, torture, murder of one’s foes, and intentional attacks upon a civilian population.

It is often difficult to embrace rules that our adversaries do not respect, and we have fought a number of wars in the last century-and-a-half against enemies who had an absolute contempt for the laws of war. But in fighting Nazi Germany, we did not ourselves become Nazis. Currently, as we combat terrorists who revel in killing innocent civilians, we again face a despicable foe but we cannot allow ourselves to become like them. What sets us apart from such enemies are our values, our fundamental beliefs, our adherence to rule of law.

By undercutting a military justice system that enforces those guiding principles, the president has not strengthened our armed forces. By sanctioning illegal behavior, he endangers the safety of our troops, giving our adversaries a green light to similarly mistreat men and women who serve honorably in our military. The president’s action encourages indiscipline and it runs counter to our values as a nation — values that long have bonded together troops of diverse races, ethnicities, cultures and religions into a unified, highly effective military force. His actions can only serve to weaken us and to strengthen our enemies.

Benjamin Freakley retired as a lieutenant general from the U.S. Army. He commanded the Tenth Mountain Division in Afghanistan. Clint Williamson served as U.S. ambassador-at-large for War Crimes Issues under Presidents Bush and Obama and earlier as an international war crimes prosecutor. He is a distinguished professor of practice at Arizona State University College of Law. Both authors serve as senior directors at the McCain Institute for International Leadership in Washington. The views expressed here are theirs alone.