They flew home from Iraq together  on a spring day in 2010 on a C-17 cargo plane, the dog in a metal kennel on the floor at the soldier’s feet. When they reached the United States, the soldier went home to his farm in Fountain, Colo., and the dog was transported to the Army’s military kennels at Fort Carson, about 12 miles north.

Sergeant 1st Class Matthew Bessler and the Belgian Malinois named Mike had been part of a canine tactical team with the 10th Special Forces Group based at Fort Carson. On the ground in Iraq, their work had been phenomenal, earning Bessler two Bronze Stars, among the most coveted commendations in the military.  During their second tour as part of an elite Special Operations group in a particularly deadly phase of the war, the pair had spent every day and night together for eight months.

Now back home, the days were never longer than when they were apart. It was as if one’s existence was proof of the other’s survival.

Both the soldier and the dog had come home with symptoms of post-traumatic stress. Mike was retiring as a combat dog, although his PTSD would get worse before it got better. And although Bessler had been in denial about his PTSD for some time, it would soon become clear that it had become too severe for him to return to war.

Bessler  had already planned to adopt Mike, but in time he would wind up training him for a new job —  this time as  a service dog to help protect Bessler from the unpredictable menace of PTSD.

“Michael is a brother,” said Bessler, who served more than half of his 20-year military career in Special Operations, with an expertise in engineering and intelligence gathering. “He needs me just as much as I need him.”

For his service, including the detection of thousands of pounds of explosives and bomb-making materials that likely no human or machine could have located, Mike had been promoted to the rank of Major. That was part of the Army’s long tradition of bestowing ranks upon war dogs; the dog’s rank was usually at least one above the soldier’s to encourage respect and discourage abuse.

To Bessler, Mike was a soldier, and their bond was as strong or stronger than the love that can grow between soldiers during combat. Bessler would lose touch with many of his battle brothers over time, but Mike would become a constant in a world spinning with chaos.

Post-Traumatic Stress Disorder, PTSD, was not a new diagnosis for soldiers returning from Iraq and Afghanistan at that time. But PTSD, traumatic brain injury and the physical and mental needs of the 2.1 million veterans of those long wars would soon become part of an all-out crisis for America.

It was no surprise that Bessler, a highly decorated Army Ranger who fought for long stretches in some of the most violent of America’s recent wars and conflicts — Somalia, Haiti, Bosnia, Iraq — could return home damaged.

But Canine PTSD was a new diagnosis, only then emerging as a possible explanation for some of the troubling behaviors some veteran combat dogs exhibit.

A key to diagnosing a war dog with post-traumatic stress is noting whether the dog’s behavior has changed in the same setting, according to Dr. Walter F. Burghardt Jr., the chief of Behavioral Medicine at the Daniel E. Holland Military Working Dogs Hospital at Lackland Airforce Base in Texas.

So if a dog consistently searches for bombs, for example, and suddenly stops in the midst of working, with no change in his environment, that would be a clear sign of trauma.  “It usually involves a situation where the dog is not working as we expect it to,” Dr. Burghardt said.  An estimated 5 to 10 percent of the 650 military working dogs who served in combat are expected to show symptoms of Canine PTSD, he said.

Looking back, Bessler believes he can pinpoint the exact moment  that pushed Mike over the edge.

After Bessler and Mike returned home from Iraq, Bessler put in the papers to adopt his wartime companion, whose official military name was K-9 Mike 5 #07-257.  While the adoption was pending, Bessler would get up at dawn and drive to Fort Carson, near Colorado Springs, to see Mike at the kennel, inside a shabby beige structure called Building 6001.

The dog was refusing to eat unless Bessler was there with him. So first Mike would eat from a bowl of dry food, and then Bessler would let the 52-pound Belgian Malinois out of his barren steel cage.

Mike would be elated as soon as he spied Bessler. The dog’s lean and muscular body would shake with anticipation — his ears perking up, his long tail wagging furiously, his giant pink tongue hanging out.

And Mike would know what was coming next: chasing and chewing on his cherished  tennis balls. The Belgian Malinois, nicknamed “the malligator,” is known for its propensity to chew and the incredible strength of its bite.  Mike was trained with the “ball reward system,” and tennis balls were his prize for doing his duty in Iraq.

During Bessler’s early morning visits to the kennel in Ft. Carson, the two would go around the side of the building to an old basketball court where Mike could chase the ball. Bessler would return to the kennels in the middle of the day to play ball, come back at night, and think of Mike during the hours in between.

One day a trainer called Bessler. Mike was refusing to work with any other handler. In effect, the dog was insisting that he stay with his human brother.

“He’s waiting for you to come back,” he told Bessler. “Come pick up your dog.”

The adoption was official, three weeks after Mike and Bessler had arrived home from Iraq.

On Bessler’s farm, Mike was surrounded by other dogs, cats, horses, chickens, a billy goat — and all the tennis balls he wanted. He was out of that grim cage at Fort Carson and enveloped by love.

But soon after Mike settled in, the dog began anxiously chewing on rocks instead of tennis balls, crushing his teeth and destroying his gums and a chunk of his lip. He was hyper and hypervigilant, unable to focus and easily spooked by loud noises. He was having accidents in the house.

Mike’s gum and lip injuries got so bad that Bessler and his ex-wife took him to the emergency room several times. A veterinarian in Colorado helped perform a series of successful surgeries to essentially reconstruct his nose and mouth.

Mike’s veterinarian in Colorado, Carin Ramsel, said that Mike’s condition met the criteria for a PTSD diagnosis, and she prescribed 20 milligrams of Prozac a day for anxiety.

Although his symptoms had become horrifyingly worse, Mike had first starting show signs of PTSD about six months into his last tour with Bessler.

Bessler and Mike had been posted at a small, makeshift base in the swamplands of Basra Province, in southeastern Iraq. Bessler recalled that he had received intel about a pocket of insurgent activity within about two miles of the post. But in order to get there, he and Mike, along with an Iraqi Intelligence officer and two other members of the 10th Special Forces Group, had to cross a river in a small inflatable boat at night.

The soldiers loaded up all their gear into the boat and made their way about halfway across the river when the boat started taking on water.  It was near the point of capsizing when Bessler and the other soldiers began throwing their rucksacks, rifles, ammo and other gear overboard.

The Iraqi intelligence officer shouted to the other soldiers that he couldn’t swim. Bessler told him not to panic. He grabbed Mike’s 15-foot leash and both he and the dog went overboard, sinking down into the river, which was filled with muck and kelp.

Bessler jumped into the water and swam toward them. He got hold of Mike first and then tried to grip the intelligence officer’s right arm and pull him along with the dog toward the shore. But the arm slipped through Bessler’s hand and the man went under.

He realized then that Mike, who was trained to rappel from an airplane and had mastered other highly advanced tasks, had no training in the water and was on the verge of drowning. But Bessler was holding onto Mike for dear life, gasping for air and pushing his way upward through the morass of kelp.

The boat was floating away. The two other soldiers had made it to shore and  the intelligence officer was dead under the water as Bessler and Mike made their way to the riverbank. Bessler followed the faint glare of a light that was flashing friendly code.

The feeling of the Iraqi intelligence officer’s arm slipping from his fingers was as haunting as any of the bad deaths Bessler had witnessed or tried to stop. But this time Mike’s life had been at stake. It had felt beyond  terrifying.

Soon after that Mike stopped searching for bombs. Instead he was jumpy, on high alert, looking around to try and keep Bessler safe but no longer sniffing for explosives, a key requirement of his job.

So Bessler took Mike to the 10th Group’s lead dog trainer in Baghdad, who spent some time with the dog and then told Bessler: “He’s done working.”

Back in Colorado, once Mike had been on Prozac for six months or so, he became calmer, more focused, more trusting, Dr. Ramsel said.  Still, like many returning veterans, Mike needed a purpose. He was a rock star in Iraq until he stopped searching for bombs. Now he was an unemployed Type A dog.

At that point Bessler was slowly coming to terms with his own disabling trauma and the fact that he would not be able to return to war.  He knew what service dogs could do for struggling veterans and decided to learn everything he could about training one. Mike has been his service dog for three years now.

Facing a recent painful and legally messy divorce, Bessler moved from Colorado back to his hometown of Powell, Wyo., where he was a star wrestler in high school and where his father and other relatives live.  The connection began so long ago, it’s no stretch to say that Mike can read Bessler’s mind –only he does it with his nose, picking up on the scents of his moods.

Now when Bessler goes to Wal-Mart, where it can feel like a minefield of overstimulation is waiting for him in every aisle, if he starts to panic he’ll walk with Mike into a corner. And Mike will stand on his feet. The physical pressure is recognized as one of many ways service dogs can help reduce anxiety.

There is also a science to Mike’s ability to help Bessler with his anxiety and fear: the hormone oxytocin, which creates feelings of safety and calm, and is stimulated in both dogs and humans when they interact with each other.

Besides PTSD, Bessler also lives with chronic headaches and migraines, shoulder and back pain — most likely from years of carrying a 60-to-70 pound rucksack — and tinnitus, a continuous, distracting and often painful ringing in the ears. He also suffers from memory and speech problems and blurred vision.

You wouldn’t know Bessler had so many medical and mental health issues from looking at him. That’s why experts say the signature wounds of these wars are mostly invisible.

Thick with muscle, heavily tattooed but with a low-key manner, Bessler could simply be the likable guy in a baseball cap riding around in his pickup truck with a cool dog and a really cute puppy in the front seat. The puppy, Ziva (a Hebrew name that means brilliance and splendor ) is a black lab that was a present from a neighbor for Valentine’s Day.

But Bessler’s moods change day-to-day and sometimes hour-by-hour. Sometimes medications work, sometimes they don’t. Out of nowhere, he’ll be overcome by a flashback. He’ll get so fed up with the nightmares and sleepless nights — “Why even bother to nap or sleep?” he said by telephone the other night — he’ll think the only thing that can stop all the physical and psychic pain is death.

Then Mike will pick up on Bessler’s depressive state, and as well-trained service dogs do, interrupt him, stop the demons from taking over. The dog will climb on top of Bessler as if to make sure his master can not go anywhere and hurt himself. Or, he’ll drop a tennis ball or a stuffed toy in Bessler’s lap and refuse to leave until he gets to chase one of them.

And Bessler will consider that taking his own life would mean leaving Mike behind. A credo in the military is to leave no soldier behind.

“That’s not being fair to the dog, not being fair to that partner who’s stood beside me forever,” he said, crying. “When you can escape yourself for a minute, and stop being selfish and think about the things you have, in my world it’s that dog.”

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