(CNN) Clarence Smoyer's index finger caressed the trigger. Sweat poured down the leather flaps of his helmet. No one in his tank moved or even whispered.

It was March 6, 1945, and Smoyer was part of the Allies' last push into Nazi Germany. The lanky 19-year-old with a mop of curly hair was part of a tank crew that had crawled into the German city of Cologne for what would become the US Army's biggest house-to-house fight in Europe. The Germans called it "Endkampf," the final battle for their homeland.

"Gentlemen, I give you Cologne," Smoyer's commander announced over the radio. "Let's knock the hell out of it!"

Smoyer didn't need any added motivation. Before he entered the shattered city, he'd received word that his cousin and his wife's brother had both been killed in the war. Those bastards are going to pay, he vowed.

Clarence Smoyer during the war. He gained a reputation for deadly accuracy as a tank gunner.

Now he intended to fulfill his promise. His M26 Pershing tank had just been engaged in a shootout with a German tank at a sprawling intersection in the town's center. But then the enemy tank ducked behind a building. Smoyer searched for it, scanning a hellish urban landscape of rubble, sagging streetcar cables and collapsed buildings.

"Staff car!" someone yelled over the radio.

A black Opel streaked into the intersection. With orders to shoot anything that moved, Smoyer pressed the trigger. Bullets and tracers from Smoyer's gun smashed into the car; ordnance from another source also flew through the intersection. The car crashed into the sidewalk, and then Smoyer saw something that made the pit of his stomach fall out.

The car's passenger door swung open and a person with a light-colored sweater embroidered with flowers crumpled to the street. He saw a flash of curly brown hair.

Smoyer's adrenalin turned to horror: Did I just shoot a woman?

The past can destroy the present

Smoyer's question would force him to return to Cologne 68 years later. It would force him to reach out to an unlikely ally. And it would force him to deal with another question that may not be limited to war veterans: How do you atone for a terrible deed when you're not sure you've committed it?

How Smoyer answered those questions is the subject of an upcoming book, "Spearhead: An American Tank Gunner, His Enemy, and a Collision of Lives in World War II." Written by Adam Makos, the engrossing book is a war story and a mystery.

Yet unlike other mysteries, this was one the main character wasn't sure he really wanted to solve -- at least not at first. Smoyer dreaded the answers because still more questions had lingered at the back of his mind for decades: Was that a woman? If so, why was she there? And perhaps most important: Did she survive?

"I often thought, 'Why the hell would somebody drive into a place like that,' " Smoyer says today.

Smoyer is now 95 and long retired from his job as a supervisor at an industrial cement plant. He was married to Melba, the woman who sent him homemade fudge when he was in combat, for 70 years before she died in 2017. They had two daughters.

Smoyer with a US tank from World War II. German tanks had Americans outgunned, but Smoyer destroyed a feared German tank during a legendary battle.

Anyone passing Smoyer on the street today wouldn't imagine that he fought in one the most legendary tank duels of World War II, says Makos. He destroyed a dreaded Panther tank, one of the most formidable weapons in the German arsenal. Yet Smoyer doesn't revel in war stories. He turns the channel when war movies come on. He closes the window when Fourth of July fireworks go off. He's a big quiet guy with a nervous chuckle.

"It was hard for me to believe that he was a legendary tank gunner in World War II because he was so gentle and calm," Makos says.

Smoyer had good reason to turn away from the sounds of war. For those who fought in tanks, war was literally hell on wheels.

This is what they faced:

Men who hadn't showered in three weeks were crammed together into small metal capsules. Shells that penetrated through tank hulls ricocheted inside like supersonic pinballs. Concussion forces from the impact of shells shattered men's bones and turned their bodies to jelly, with just the skin holding their corpses together. Tanks were called "crematoriums on wheels."

"The mechanics and maintenance men used to cry when they came out from cleaning a tank," Makos says.

Smoyer (top center) and his crew pause during a lull in the fighting. He thought he wouldn't survive the war.

But repressing those memories could also be lethal.

Smoyer had seen it happen before.

In one of the most heartbreaking scenes in the book, Makos tells the story of Smoyer's company commander, Capt. Mason Salisbury.

Salisbury had returned home from the war, graduated from Columbia Law School and become a lawyer at big New York firm.

One winter weekend, he stayed at his parents' mansion on Long Island. He played tennis and ate dinner with his family. That Monday morning, his father found him slumped in a car in the garage. The car had been running all night. Salisbury had killed himself at 30.

He'd left a note, saying he was depressed over the loss of his friends in battle.

Makos wrote:

"After surviving more close calls with German shells than he could count, Capt. Salisbury had been stalked and cut down by the unseen killer. The mental toll of war."

Smoyer discovers a vital clue

Smoyer was losing the battle with that unseen killer. The mental toll of the war was eating into him.

He'd become a hero for his actions in Cologne that day. After his encounter in the intersection, his tank would go on to tangle with another German tank known as the "monster" -- the dreaded Panther. It had a gun so powerful that its shells could go through one American Sherman tank into the next.

Tank warfare was so grisly during World War II that American soldiers had a nickname for tanks: crematoriums on wheels.

Smoyer and his crew destroyed that Panther, and the duel was caught on film by a combat cameraman. Journalists heard about the battle and interviewed and photographed Smoyer and his crew. One journalist wrote a piece about it called, "Killing a Monster."

But as the years went by, Smoyer tried not to think about the war. He'd have flashbacks of battles, but he pushed those memories away. Yet one question never quite went away: What really happened at that intersection?

One day he got some answers. They came in his mailbox.

A war buddy had sent him a VHS tape called, "Scenes of War." It turned out that a combat cameraman had also filmed what happened in the intersection.

Smoyer plopped the tape into his video player and started reliving a battle he had fought over 50 years earlier.

He saw the massive intersection again. The rubble. The car streaking through as bullets and tracers from his tank and the German tank zipped across the roadway. He saw the car crash again, a body tumbling out of the passenger side, a familiar flash of curly brown hair.

It was a woman.

He saw her stare vacantly into the sky as American medics tried to treat her. He saw her curl into a fetal position as one medic tenderly covered her with a blanket. He saw her blink as soldiers walked by.

The woman from the car fights for her life as American medics tend to her wounds.

His chest started to heave as he realized what he had done. At the time of the battle, he was too preoccupied with surviving the war to dwell on his actions. And he kept the memories at bay for years afterward. Now, he could no longer do that.

"I had forgotten about it for decades, the car was just a blur, and now the whole thing came back, clear as day," Smoyer says.

Smoyer started having nightmares. He'd wake up swinging, afraid he'd hit his wife, Melba. He had to take medication to calm himself, but the nightmares kept coming. He could barely function in the day. He kept seeing the woman in his dreams.

The past was destroying his present.

"The war is over, I thought, why do I have to worry about this anymore?"

Smoyer couldn't change what he'd seen, but maybe he could lessen its sting. He acquired uncut footage of the battle from the National Archives and, with notepad in hand, scoured the film for another explanation.

"I prayed that I wasn't the one," Smoyer says.

Doubts and denial crept into his mind. Maybe it wasn't his fault? Maybe the German tank had shot the woman?

"He was searching for a way he didn't do it," Makos says.

None of his war buddies could satisfy that search. The rest of his tank crew had all died of old age. Then another idea struck Smoyer. What if he could contact the German tank gunner?

What if he was still alive?

An enemy becomes a comrade

Gustav Schaefer stood in a Cologne square on a frigid winter afternoon in March 2013, arms tucked behind his back, wondering why an American soldier wanted to talk to him.

Who was this Clarence Smoyer?

Schaefer was the gunner in the tank that had faced Smoyer 68 years earlier. Barely 5 feet tall, he'd been a teenager assigned to a tank because of his size.

Gustav Schaefer was the German gunner who faced Smoyer in the fight for Cologne. They shared a battle and a common nightmare.

A Cologne journalist had contacted Schaefer, saying Smoyer wanted to meet him. Schaefer agreed but was nervous: What did he come all the way to Germany to talk about? Would he be angry?

Smoyer spotted Schaefer and walked toward him, picking up the pace as he got closer. Smoyer reached out with an open hand as Schaefer timidly extended his.

What Smoyer said next put Schaefer at ease:

"The war is over and we can be friends now."

Schaefer hadn't been the prototypical Nazi soldier, Makos said. He was a farm boy from Northern Germany who'd been drafted. He actually grew up admiring the United States from afar, reading about "cowboys and Indians" and learning about Mickey Mouse. He said he had no animosity toward Jews. A Jewish neighbor had loaned his family a car once when they'd hit hard times and had never asked for anything in return.

"They didn't have electricity. He didn't have a radio. He only had a few books," Makos says. "His big enjoyment was riding his bike to the train tracks to watch the trains go by."

Now Schaefer was about to relive a painful drama with Smoyer.

After pleasantries, they retreated to a hotel and shared evening beers. Smoyer told him why he'd sought him out. The next day they decided to confront their past. Together they walked to the scene where the car crashed, where the woman's body tumbled out.

Makos was there to record what happened next.

"This is where I see her in my dreams," Smoyer told Schaefer, pointing to a lamppost.

American medics try to help the woman after her car was caught in the crossfire. This is the sight that stayed with Smoyer for years.

Schaefer knew what Smoyer meant. He had seen the same film that Smoyer saw while watching a television documentary a decade earlier. He told Smoyer that he, too, kept having nightmares about the woman.

They began to talk as two old soldiers, recalling the battle.

Smoyer said the intersection was a shooting gallery. He didn't have time to study the car. And that's when Schaefer said something that filled in the blanks for Smoyer:

"Well, that's why I shot it, too."

It turns out that both he and Schaefer had shot at the car. Smoyer no longer bore his guilt alone. Another soldier had reacted just as he had. It's what they were trained to do.

Smoyer's eyes teared up. Then he became angry. Why, he wondered, would a civilian drive into the middle of a battle zone?

"It's war," Schaefer said. "It's in the nature of it. It can't be undone."

Smoyer, though, had his own surprise for Schaefer. He had learned who the woman was and what had happened to her.

Now Smoyer and Schaefer were about to pay their respects to her.

Both started walking.

The woman in the car

Her name was Katharina Esser, and she was 26. The youngest of four sisters, she was called "Kathi." All her sisters had married and had families. She stayed home to care for her parents, and would often take her nieces and nephews to the park, pushing them on their scooters.

Kathi Esser, the woman caught in the crossfire, was a caretaker for her family. She looked out for her parents and cared for her nieces and nephews.

Esser took night classes to get a degree in home economics. She worked as a clerk in a grocery store. The driver of the car that day was her boss, the owner of the grocery. Makos suspects that Esser and her boss had gone stir crazy staying in an air raid shelter and had decided to make a run for a bridge outside Cologne to safety.

All three of Esser's sisters had lost their husbands in the war.

"Life only has these (sad) things to offer us nowadays," Esser wrote in a letter to one of her family members after learning that one of her brothers-in-law had been killed in battle. "I don't believe in a good outcome anymore."

Esser, though, still had something to look forward to.

"She hoped that she would be a mother one day and have children of her own," Makos says.

Instead, Esser died after being caught in the crossfire. She was buried in a church cemetery, just 200 yards from where she had fallen.

Smoyer and Schaefer walked along a path to the church and stopped before a knee-high wooden cross. A plaque on the cross read, "The Unknown Dead." Esser had been buried in a mass grave. At the time, people weren't certain of her identity. Her papers had been separated from her at the time of her death. But Germans were methodical record keepers. And one of her sisters had seen the film of her death and realized it was Kathi.

After writing journalists and historians in Cologne, Smoyer had pieced together her identity and her final resting place.

Now Schaefer knew, too. Both placed yellow roses on the grave. As Smoyer bent down to place his rose, he almost lost his balance. Schaefer grabbed his arm and steadied him.

Smoyer told Esser he was sorry.

"Visiting the gravesite gave me a chance to apologize to Katharina, just me and her," Smoyer says.

The fate of Kathi Esser haunted the American and German soldiers who saw her fall before their tanks during the last days of the war.

Later on the trip, Smoyer got another helping hand -- from Esser's family.

They heard he was in town and invited him to their house. They told him to be at peace and that their Kathi wouldn't have blamed him. One of Esser's nieces told him:

"The people who started this war are the ones who killed Katharina."

Smoyer says the meeting with Esser's family "brought me some comfort."

"War is hell," he says. "No matter what side you stand on. A lot of young people get killed, but it's the leaders of the countries who should have to do the fighting on the front lines. If that happened I'm sure there wouldn't be wars anymore."

Smoyer remained friends with Schaefer afterward, exchanging letters and talking on Skype. Schaefer died in 2017. Smoyer sent a bouquet of flowers to his funeral with the inscription:

"I will never forget you! Your brother in arms, Clarence."

And both he and Schaefer never forgot the woman in their dreams. They said farewell to Kathi.

They used their past to save their present.

"You could write a textbook on how they healed from trauma," Makos says. "They went to the scene and confronted what had happened. They paid their respects to her and apologized. And they honored her by trying to understand who she was."

The memories of tank warfare were so bad for Smoyer that he would turn off the television when war movies came on.

Smoyer and Schaefer didn't just share their story for themselves, Makos says. They did it for Esser.

"They said we're not going to tell our war story. We're going to tell you about her. By doing so she could live forever."

It would be easy to imagine that Smoyer never sees Kathi in his dreams anymore.

Yet that is not the nature of war.

"It still hurts," he says. "It don't go away."

But he says he rests easier at night now.

"I don't wake up swinging my arms anymore and I can sleep a full night," he says. "I still see her in my dreams. I think I always will. I don't think she haunts me. It's different than that. It's not a nightmare anymore."