Myeshia Johnson is presented with the U.S. flag that was draped over the casket of her husband, Army Sgt. La David Johnson, during his burial service at Fred Hunter's Hollywood Memorial Gardens in Hollywood, Fla. (Matias J. Ocner/Miami Herald/AP)

Gerald Merna served 22 years in the Marine Corps.

And his last job was probably the toughest.

It wasn’t his time as a platoon sergeant during the bitter, muddy and bloody battle for Outpost Vegas during the Korea War. And it wasn’t his 13 months of combat duty at Phu Bai in Vietnam.

Nope. It was the year he was at Joint Base Andrews, serving as one of five casualty notification officers (CNOs) — the guys who knocked on your door with the bad news.

It’s an art, really, knowing the right thing to say as the bearer of the worst possible news.

The Procedures for the Army Casualty Program of 2015 reminds the officers they cannot be too personal and they cannot give out too much information.

They need to remember to leave their script in the car and turn their cellphones off. They shouldn’t touch the family members, they shouldn’t rush through the notification. For children, speak to them at their level, eye to eye. And never, ever use the term “passed away.”

“Always use definitive terms such as ‘killed,’ ‘died,’ or ‘dead’ when making the notification,” says Subsection G on Page 23 of the casualty manual.

Training manuals? Is the art of human compassion something to find in the subsection of a military procedure manual?

“You know how much training I had?” Merna, 87, asked. “Zero.”

“There’s no science to this,” he said.

“And you not only had to go out and tell them. We lived with them, stayed with them through the arrangements, we got their medals,” he said. “It’s more than just making a call. It was one of the toughest things I did and one of the reasons I retired early.”

Merna stopped our phone conversation to turn off the Fox News Channel in his Arlington, Va., home.

“It’s all that’s on the news, this thing,” he said, referring to the controversy over President Trump and his conversation with Myeshia Johnson, the widow of Army Sgt. La David Johnson, one of the Green Berets killed in an ambush this month in Niger.

Myeshia Johnson said the president struggled to remember her husband’s name, and she said his clumsy condolence made her staggering loss even more bitter.

“The president said, ‘He knew what he signed up for, but it hurts anyway,’ ” she said in an interview with ABC News’s George Stephanopoulos.

Merna said he knows how hard it is to say the right thing. It’s a horrific job to get just right. And he’ll agree that the president’s words weren’t the right ones, certainly not what he or his fellow officers would’ve said. (“But I think his heart was in the right place,” he said.)

CNOs rarely speak out. But when they do, nearly all of them say the same thing — it was the hardest part of their careers.

Merna remembers the families who cried, the ones who got quiet, the ones who screamed, the ones who got violent.

“There’s no two alike, no two families react the same,” he said.

When he got the assignment in 1968, he was determined to be personal, compassionate and helpful during his toughest year. Because he also remembered what it was like when his 19-year-old brother was killed in World War II.

“A telegram was it,” he said. And then a small notification in the local paper.

An officer eventually sent a letter to his mom, with details about his service and anecdotes from his brothers in arms. But that first notice never had the human touch.

“When I started doing it, we had four hours to get to the family,” he said. “It was one of the most humane things the military did.”

Embracing humanity is essential to the task. He said he’ll never forget the family gathering he interrupted in Maryland — a half-dozen family members there, cousins and uncles.

“A first cousin got so angry, he hit me,” Merna said. “And I understood that.”

The man calmed down, apologized and shook Merna’s hand.

“He told me, ‘I’m sorry, sir,’ ” Merna said. “And I told him I was sorry, also.”

Eventually, the Defense Department came up with protocols, scripts and training.

Merna’s advice?

“Throw it all away,” he said. “Forget the rule books.”

“What it’s really all about is common sense, use your feelings. Go in there with what’s in your heart.

“Pretend that’s your mom you’re talking to. Think of how you react if someone knocked on your door with that news.”

Merna knew how to take a punch, how to have humility and humanity when facing a fellow American’s lowest moment. That’s called leadership. We could use more of it.

Twitter: @petulad