(CNN) The pilot glanced outside his cockpit and froze. He blinked hard and looked again, hoping it was just a mirage. But his co-pilot stared at the same horrible vision.

"My God, this is a nightmare," the co-pilot said.

"He's going to destroy us," the pilot agreed.

The men were looking at a gray German Messerschmitt fighter hovering just three feet off their wingtip. It was five days before Christmas 1943, and the fighter had closed in on their crippled American B-17 bomber for the kill.

The B-17 pilot, Charles Brown, was a 21-year-old West Virginia farm boy on his first combat mission. His bomber had been shot to pieces by swarming fighters, and his plane was alone in the skies above Germany. Half his crew was wounded, and the tail gunner was dead, his blood frozen in icicles over the machine guns.

But when Brown and his co-pilot, Spencer "Pinky" Luke, looked at the fighter pilot again, something odd happened. The German didn't pull the trigger. He nodded at Brown instead. What happened next was one of the most remarkable acts of chivalry recorded during World War II. Years later, Brown would track down his would-be executioner for a reunion that reduced both men to tears.

Living by the code

People love to hear war stories about great generals or crack troops such as Seal Team 6, the Navy unit that killed Osama bin Laden. But there is another side of war that's seldom explored: Why do some soldiers risk their lives to save their enemies and, in some cases, develop a deep bond with them that outlives war?

And are such acts of chivalry obsolete in an age of drone strikes and terrorism?

Charles Brown was on his first combat mission during World War II when he met an enemy unlike any other.

Those are the kinds of questions Brown's story raises. His encounter with the German fighter pilot is beautifully told in a New York Times best-selling book, "A Higher Call." The book explains how that aerial encounter reverberated in both men's lives for more than 50 years.

"The war left them in turmoil," says Adam Makos, who wrote the book with Larry Alexander. "When they found each other, they found peace."

Their story is extraordinary, but it's not unique. Union and Confederate troops risked their lives to aid one another during the Civil War. British and German troops gathered for post-war reunions; some even vacationed together after World War II. One renowned American general traveled back to Vietnam to meet the man who almost wiped out his battalion, and the two men hugged and prayed together.

What is this bond that surfaces between enemies during and after battle?

It's called the warrior's code, say soldiers and military scholars. It's shaped cultures as diverse as the Vikings, the Samurai, the Romans and Native Americans, says Shannon E. French, author of "Code of the Warrior."

The code is designed to protect the victor, as well as the vanquished, French says.

"People think of the rules of war primarily as a way to protect innocent civilians from being victims of atrocities," she says. "In a much more profound sense, the rules are there to protect the people doing the actual fighting."

The code is designed to prevent soldiers from becoming monsters. Butchering civilians, torturing prisoners, desecrating the enemies' bodies -- are all battlefield behaviors that erode a soldier's humanity, French says.

The code is ancient as civilization itself. In Homer's epic poem, "The Iliad," the Greek hero Achilles breaks the code when his thirst for vengeance leads him to desecrate the body of his slain foe, the Trojan hero Hector.

He's going to destroy us! Charles Brown, B-17 bomber pilot

Most warrior cultures share one belief, French says:

"There is something worse than death, and one of those things is to completely lose your humanity."

The code is still needed today, French says.

Thousands of U.S. soldiers returning from wars in Iraq and Afghanistan are struggling with post-traumatic stress disorder. Some have seen, and have done, things that are unfathomable.

A study of Vietnam veterans showed that those who felt as if they had participated in dishonorable behavior during the war or saw the Vietnamese as subhuman experienced more post-traumatic stress disorder, French says.

Drone warfare represents a new threat to soldiers' humanity, French says.

The Pentagon recently announced it would award a new Distinguished Warfare Medal to soldiers who operate drones and launch cyberattacks. The medal would rank above the Bronze Star and Purple Heart, two medals earned in combat.

At least 17,000 people have signed an online petition protesting the medal. The petition says awarding medals to soldiers who wage war via remote control was an "injustice" to those who risked their lives in combat.

Outgoing Defense Secretary Leon Panetta defended the new medal at a February news conference.

"I've seen firsthand how modern tools, like remotely piloted platforms and cybersystems, have changed the way wars are fought," Panetta says. "And they've given our men and women the ability to engage the enemy and change the course of battle, even from afar."

Still, critics ask, is there any honor in killing an enemy by remote control?

French isn't so sure.

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"If [I'm] in the field risking and taking a life, there's a sense that I'm putting skin in the game," she says. "I'm taking a risk so it feels more honorable. Someone who kills at a distance -- it can make them doubt. Am I truly honorable?"

The German pilot who took mercy

Revenge, not honor, is what drove 2nd Lt. Franz Stigler to jump into his fighter that chilly December day in 1943.

Stigler wasn't just any fighter pilot. He was an ace. One more kill and he would win The Knight's Cross, German's highest award for valor.

Yet Stigler was driven by something deeper than glory. His older brother, August, was a fellow Luftwaffe pilot who had been killed earlier in the war. American pilots had killed Stigler's comrades and were bombing his country's cities.

Stigler was standing near his fighter on a German airbase when he heard a bomber's engine. Looking up, he saw a B-17 flying so low it looked like it was going to land. As the bomber disappeared behind some trees, Stigler tossed his cigarette aside, saluted a ground crewman and took off in pursuit.

As Stigler's fighter rose to meet the bomber, he decided to attack it from behind. He climbed behind the sputtering bomber, squinted into his gun sight and placed his hand on the trigger. He was about to fire when he hesitated. Stigler was baffled. No one in the bomber fired at him.

He looked closer at the tail gunner. He was still, his white fleece collar soaked with blood. Stigler craned his neck to examine the rest of the bomber. Its skin had been peeled away by shells, its guns knocked out. He could see men huddled inside the plane tending the wounds of other crewmen.

Then he nudged his plane alongside the bomber's wings and locked eyes with the pilot whose eyes were wide with shock and horror.

Franz Stigler wondered for years what happened to the American pilot he encountered in combat.

Stigler pressed his hand over the rosary he kept in his flight jacket. He eased his index finger off the trigger. He couldn't shoot. It would be murder.

Stigler wasn't just motivated by vengeance that day. He also lived by a code. He could trace his family's ancestry to knights in 16th century Europe. He had once studied to be a priest.

A German pilot who spared the enemy, though, risked death in Nazi Germany. If someone reported him, he would be executed.

Yet Stigler could also hear the voice of his commanding officer, who once told him:

"You follow the rules of war for you -- not your enemy. You fight by rules to keep your humanity."

Alone with the crippled bomber, Stigler changed his mission. He nodded at the American pilot and began flying in formation so German anti-aircraft gunners on the ground wouldn't shoot down the slow-moving bomber. (The Luftwaffe had B-17s of its own, shot down and rebuilt for secret missions and training.) Stigler escorted the bomber over the North Sea and took one last look at the American pilot. Then he saluted him, peeled his fighter away and returned to Germany.

"Good luck," Stigler said to himself. "You're in God's hands."

What creates the bond between enemies?

Stigler was able to recognize the common humanity of the enemy when he locked eyes with Brown. It caused him to take mercy.

That sudden recognition can spring from many sources in battle -- hearing the moans of a wounded enemy; sharing a common language; or opening the wallet of an enemy and seeing pictures of his wife and children.

That respect for the enemy's humanity typically starts at the top, some scholars say. A leader sets the tone, and the troops get the message. A military leader who embodied this approach was one of Germany's greatest World War II commanders, Field Marshal Erwin Rommel, also known as the "Desert Fox."

One time, a group of British commandos tried to sneak behind enemy lines and assassinate Rommel in the North African desert. They failed. But Rommel insisted the commandos be buried in the same graveyard as the German soldiers who died defending him, says Steven Pressfield, author of "Killing Rommel."

There were battle zones during World War II where that type of magnanimity was almost impossible. On the Eastern Front, German and Russian soldiers literally hated one another. And in the South Pacific, U.S. Marines and Japanese soldiers took no prisoners.

At times, the terrain can force soldiers to follow the code. The North African desert during World War II was one such place, Pressfield says.

Fortunes turned quickly because so many battles were fought by fast-moving tanks and mobile units. A German unit that captured British soldiers could end up surrendering to them minutes later because the battle lines were so fluid. Also, the desert sun was so harsh that both sides knew if they left enemy prisoners stranded or mistreated, they would quickly die, Pressfield says.

In many ways, a soldier feels more of a bond with the enemy they're fighting than with the countrymen back home. The enemy they're fighting is equally risking death. Steven Pressfield, author of "The Warrior Ethos"

It was not unusual for German and British doctors to work together while taking care of wounded soldiers from both sides, Pressfield says.

Some British and German soldiers never forgot how their enemy treated them and staged reunions after the war.

"The Germans and the British used to get together for soccer matches," Pressfield says. "It was the Desert Foxes versus the Desert Rats."

These soldiers weren't just engaging in nostalgia. They shared a sense of hardship. They had survived an ordeal that most people could not understand.

"In many ways, a soldier feels more of a bond with the enemy they're fighting than with the countrymen back home," Pressfield says. "The enemy they're fighting is equally risking death."

That bond could even lead to acts of loyalty after the war, says Daniel Rolph, author of "My Brother's Keepers."

Once, when a Union officer mortally wounded a Confederate captain during the Civil War, the Union man sang hymns and prayed with his enemy as the man took his last breaths. Before the captain died, he asked the Union officer to return his sword and revolver to his family -- a request the soldier honored after the war ended, Rolph says.

"I even have an article from The New York Times in 1886 where Union soldiers who were on the pension rolls of the federal government were actually trying to transfer their money toward Confederate soldiers," Rolph says.

These bonds can even form between enemies who do not share a language or a culture.

Harold Moore Jr. was a U.S. Army colonel who led a desperate fight depicted in the 2002 Mel Gibson film, "We Were Soldiers Once ... And Young. " In 1965, Moore lost 79 of his men fighting against a larger North Vietnamese force. It was one of the first major battles in the Vietnam War.

In 1993, Moore led some of his soldiers back to Vietnam to meet their former adversaries on the same battlefield. When they arrived, Moore met the Vietnamese officer who led troops against him, Lt. Gen. Nguyen Huu An.

Charles Brown, with his wife, Jackie (left), found peace after his reunion with Franz Stigler, with his wife, Hiya.

An held out his arms and greeted Moore by kissing him on both cheeks. Moore gave him his wristwatch as a token of friendship.

Moore described in an essay what happened next:

"I invited all to form a circle with arms extended around each other's shoulders and we bowed our heads. With prayer and tears, we openly shared our painful memories."

An died two years after meeting Moore. Moore traveled to Vietnam to pay his respects to his former enemy's family. While visiting their home, Moore spotted a familiar object displayed in the viewing case of An's family shrine: It was his wristwatch.

A reunion of enemies

As he watched the German fighter peel away that December day, 2nd Lt. Charles Brown wasn't thinking of the philosophical connection between enemies. He was thinking of survival.

He flew back to his base in England and landed with barely any fuel left. After his bomber came to a stop, he leaned back in his chair and put a hand over a pocket Bible he kept in his flight jacket. Then he sat in silence.

Brown flew more missions before the war ended. Life moved on. He got married, had two daughters, supervised foreign aid for the U.S. State Department during the Vietnam War and eventually retired to Florida.

Late in life, though, the encounter with the German pilot began to gnaw at him. He started having nightmares, but in his dream there would be no act of mercy. He would awaken just before his bomber crashed.

Brown took on a new mission. He had to find that German pilot. Who was he? Why did he save my life?

The war left them in turmoil. When they found each other, they found peace. Adam Makos, co-author of "A Higher Call"

He scoured military archives in the U.S. and England. He attended a pilots' reunion and shared his story. He finally placed an ad in a German newsletter for former Luftwaffe pilots, retelling the story and asking if anyone knew the pilot.

On January 18, 1990, Brown received a letter. He opened it and read:

"Dear Charles, All these years I wondered what happened to the B-17, did she make it or not?"

It was Stigler. He had had left Germany after the war and moved to Vancouver, British Columbia, in 1953. He became a prosperous businessman. Now retired, Stigler told Brown that he would be in Florida come summer and "it sure would be nice to talk about our encounter."

Brown was so excited, though, that he couldn't wait to see Stigler. He called directory assistance for Vancouver and asked whether there was a number for a Franz Stigler. He dialed the number, and Stigler picked up.

"My God, it's you!" Brown shouted as tears ran down his cheeks.

Brown had to do more. He wrote a letter to Stigler in which he said: "To say THANK YOU, THANK YOU, THANK YOU on behalf of my surviving crewmembers and their families appears totally inadequate."

The two pilots would meet again, but this time in the lobby of a Florida hotel.

One of Brown's friends was there to record the summer reunion. Both men looked like retired businessmen: they were plump, sporting neat ties and formal shirts. They talked about their encounter in a light, jovial tone.

The mood then changed. Someone asked Stigler what he thought about Brown. Stigler sighed and his square jaw tightened. He began to fight back tears before he said in heavily accented English:

"I love you, Charlie."

Years later, author Makos says he understands why Stigler experienced such a surge of emotions.

Stigler had lost his brother, his friends and his country. He was virtually exiled by his countrymen after the war. There were 28,000 pilots who fought for the German air force. Only 1,200 survived, Makos says.

"The war cost him everything," Makos says. "Charlie Brown was the only good thing that came out of World War II for Franz. It was the one thing he could be proud of."

The meeting helped Brown as well, says his oldest daughter, Dawn Warner.

They met as enemies but Franz Stigler, on left, and Charles Brown, ended up as fishing buddies.

Brown and Stigler became pals. They would take fishing trips together. They would fly cross-country to each other homes and take road trips together to share their story at schools and veterans' reunions. Their wives, Jackie Brown and Hiya Stigler, became friends.

Brown's daughter says her father would worry about Stigler's health and constantly check in on him.

"It wasn't just for show," she says. "They really did feel for each other. They talked about once a week."

As his friendship with Stigler deepened, something else happened to her father, Warner says:

"The nightmares went away."

Brown had written a letter of thanks to Stigler, but one day, he showed the extent of his gratitude. He organized a reunion of his surviving crew members, along with their extended families. He invited Stigler as a guest of honor.

During the reunion, a video was played showing all the faces of the people that now lived -- children, grandchildren, relatives -- because of Stigler's act of chivalry. Stigler watched the film from his seat of honor.

"Everybody was crying, not just him," Warner says.

Stigler and Brown died within months of each other in 2008. Stigler was 92, and Brown was 87. They had started off as enemies, became friends, and then something more.

Makos discovered what that was by accident while spending a night at Brown's house. He was poking through Brown's library when he came across a book on German fighter jets. Stigler had given the book to Brown. Both were country boys who loved to read about planes.

Makos opened the book and saw an inscription Stigler had written to Brown:

In 1940, I lost my only brother as a night fighter. On the 20th of December, 4 days before Christmas, I had the chance to save a B-17 from her destruction, a plane so badly damaged it was a wonder that she was still flying.

The pilot, Charlie Brown, is for me, as precious as my brother was.

Thanks Charlie.

Your Brother,

Franz