Editor's Note: This is the fourth in a five-part series.

It was a beautiful day. Twenty-two-year-old Marine Sergeant Steve Reichenbach was working his way up a hill, his scout dog Major moving along beside him.

In fact, he wasn't supposed to go out on another mission. It was supposed to be his last day in Vietnam, his last day with Major. The replacement handlers were already in country, ready to pick up the leashes as soon as Reichenbach and his fellow handlers left Vietnam to make their way back home. But when the request came in for a dog team to go out with a group of Marines on a patrol mission, Reichenbach said he'd go—to work once more with Major.

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At 90 pounds, Major—a Great Dane-Shepherd mix with a creamy off-white coat—had an intimidating presence. After they first started running missions together, Reichenbach began to notice that once the enemy saw his dog coming, they would start tripping their ambushes early. Major's size alone was enough to scare them off.

Unlike most of the other Marine dog handlers who had been sent to Vietnam in 1966, Reichenbach hadn't trained with Major before deploying. Instead he had been paired up with the dog once he arrived in country. The handler Major had come with to Vietnam had been killed a few weeks before Reichenbach arrived. But despite the fact they were paired out of convenience, Reichenbach and Major meshed from their first meeting.

The young man and this dog had similar temperaments: They were both mellow, relaxed, even-keeled types who didn't waste much energy getting excited about much of anything. A quiet dog, Major never barked, never growled. He was never ruffled by the noise of the fighting around him. Reichenbach never saw Major out of sorts, except for the time they came across a cat—a kitten that weighed no more than three pounds. But as soon as Major caught wind of this little creature, he went crazy, moving so fast to chase the cat he nearly jerked his handler to the ground.

On their last day together, they marched along up the hill, which was mostly bare, offering no brush or trees for cover. After a while, one of the Marines in the company stepped into a booby trap—a deep pit lined with sharp spears—and one of the spears went through his boot and into his foot. While the medic was attending to the wounded man, Reichenbach turned around to walk away with Major and was hit with a bad feeling—suddenly he just knew that something wasn't right.

And then a mine exploded. A tail of shrapnel sprayed out behind it, catching six of the men and killing four of them. Reichenbach was hit in his upper right leg and left hip, his wounds bleeding freely.

And then, the dog that never growled, that was never put off by the sound of snapping gunfire or artillery shells, planted himself at Reichenbach's side and bared his teeth. Major would not let anyone come near his handler.

The other men, working fast to get Reichenbach medical attention, finally got a muzzle on the dog. The company commander, Captain Walter Boomer, hoisted the large dog up and put him on the chopper, right on top of Reichenbach. As the chopper descended back at the base, the first thing the waiting medics saw was a big white dog bearing down on them.

It was the last time Reichenbach would see Major. The handler spent the next three months recuperating in a series of different military hospitals before finally returning home to the United States. Meanwhile, Major was immediately paired up with the replacement handler. And, as one of Reichenbach's fellow Marines would tell him later, when this new handler went to meet his new dog, Major was still covered in Reichenbach's blood.

View Images Reichenbach was paired with Major, a Great Dane-Shepherd mix, when he arrived in Vietnam, soon after the dog's first handler was killed. PHOTOGRAPH COURTESY OF STEPHEN K. REICHENBACH

Reichenbach never had another dog. After the war ended, he didn't try to track down Major the way that some handlers did, sending inquiries after their dogs, hoping to adopt them (all unsuccessfully). Someone sent him an email once, saying they'd found a record noting that Major died of a jungle disease that had been killing off their dogs. But even if Major was still alive by the time the United States forces pulled out of Vietnam, he, like all but a few of the dogs still in country, would have been left behind.

And many of these military dogs met with an unhappy end—likely euthanized by the South Vietnamese Army, with whom they were left, or worse. Many of the handlers didn't find out for years that their canine partners never made it out of Vietnam alive.

This is one of the darkest parts of war dog history, especially considering how valuable they were to U.S. troops. Roughly 4,000 dogs served in the war, leading patrols with their handlers through dense jungle terrain. Overall, they are credited with saving upward of 10,000 lives.

After he got out of the Marine Corps, Reichenbach never had another dog, but he still thinks of Major. Whenever some website asks for the name of his first pet as a security question, Reichenbach always lists Major, even though he wasn't really his first dog or really a pet. He was something more.

"He was a good puppy," he says. "He deserved better than he got. But," he pauses, thinking for a moment, "it was a useful life."

Coming tomorrow: Smoky, the Healing Dog