When Felix Sparks entered the Dachau concentration camp at the end of April 1945, his men were already enraged. The 45th Infantry Division was still reeling from the discovery of 39 boxcars containing thousands of corpses outside the camp, and from hearing the accounts of surviving inmates of the atrocities they had suffered since the camp had opened, in 1933.

Just 26 years old, Sparks had joined the Army to make money for law school. He, like many soldiers, had never even heard of concentration camps. But by the time Sparks reached Dachau, he was already a war hero. In 1944’s Battle of Anzio, which ended with the Allied capture of Rome, almost his entire company had been killed. When they got to Dachau, Sparks didn’t expect “much of a battle,” according to his later testimony to the Shoah Foundation — and he didn’t get one. At least not at first.

Sparks believed high-ranking Nazis at the concentration camp, including the camp commander, had fled ahead of the expected Allied incursion. In the days before American forces reached Dachau, thousands of prisoners were forced out of the camp and on a death march by the Nazis. When Sparks arrived, some 30,000 prisoners remained. According to Harold Marcuse, author of Legacies of Dachau, replacement leader Heinrich Wicker had roughly 560 personnel at his disposal, including Hungarian troops and conscripted inmates from a disciplinary prison inside the camp.

After Sparks and his men scaled the Dachau’s 8-foot walls, they rounded up approximately 50 German soldiers and lined them up against a wall, with a few armed U.S. soldiers guarding them. Sparks watched them until a member of his division called him over to “see what we found.” That’s when he heard the gunfire and ran back to where the POWs were.

Sparks kicked the soldier who’d fired off 20 rounds, then grabbed him by the collar. “What the hell are you doing?” he demanded. The soldier maintained that the prisoners had tried to escape, but Sparks didn’t believe it. His men, however, kept firing.

“I ran over there, I drew my … .45 and fired it into the air several times to get everybody’s attention. I was yelling that there should be no more firing unless I give a specific order,” Sparks later testified.

“[This incident] shows an ordinary working-class American GI does not let his man become like the enemy, become beasts,” Alex Kershaw, author of the Sparks biography The Liberator, said during a discussion at the Friends of the National World War II Memorial. “He stops them. He shows in that one moment that he has integrity, a moral core and courage that allows him to do the right thing at the most difficult time.”

However, Marcuse says the situation on the ground was likely more complicated. “There’s a sort of military code of honor about how to treat prisoners of war,” he says, “but the Germans violated it openly.” In December 1944, during the Battle of the Bulge, 84 American prisoners had been mowed down with machine guns by German soldiers.

While Sparks maintained his own moral code, he acknowledged the horrors of what his men had seen. “We were used to death. I mean, death was a constant companion,” he later said. “But we never witnessed anything of this magnitude.” After the liberation, the camp’s former prisoners hunted down their torturers with any weapons they could find, and American soldiers rarely tried to restrain them. None of the Americans who shot guards that day were ever tried in court. Still, historians dispute later accounts of the liberation that describe the killings of the guards as a massacre.

After the war, Sparks returned to the Southwest and settled in Colorado, where he headed the state National Guard as a brigadier general. He also attended law school and eventually made it onto the Colorado Supreme Court.

But Sparks’ battle against violence didn’t end with the war. In 1993, his grandson was killed in a shooting, and once again, Sparks — then 76 — suited up for combat, going toe-to-toe with the National Rifle Association to demand better legislation against gun violence. He got it: A state law banning minors from carrying handguns was passed. “The thing about war is it can give you a pretty low opinion of mankind,” Sparks told the Rocky Mountain News in 1993. “I don’t have a low opinion of mankind, but sometimes we sure do some stupid things.”