But in November, Mr. Bojorquez got a text from Mr. Guerrero that upended everything. “I don’t think I can do this life anymore,” it said.

Mr. Guerrero had never mentioned it to others, but he still believed his sergeant’s death was his fault. If only he had yelled a warning. Or spotted the I.E.D. He was getting therapy and medication for his depression, but still often woke up with a deep dread, as if he were sitting at the principal’s office, waiting to be punished. Every day, he wore a bracelet etched with the sergeant’s name.

That night, Mr. Guerrero had been watching television with his wife after church when something snapped. He crumpled to the floor and backed into a corner, crying, “I’m sorry, I’m sorry.”

He had not smoked since the Marines, but pleaded with his wife to go out and buy cigarettes. The panic and guilt were so excruciating that he decided the only relief was to kill himself. He went onto his porch with shaking hands to text Mr. Bojorquez to say goodbye.

Mr. Bojorquez called immediately. Mr. Guerrero picked up, sobbing, but after a few words hung up.

A fear had crept over Mr. Bojorquez over the last year that he was doomed to watch his friends die one after another until he was the only one left. At times, he saw it as another reason to kill himself. But it was also motivation to break the pattern.

He knew he had to call 911, but hesitated. The call might land Mr. Guerrero in a psychiatric ward or ruin his marriage, already strained. Worse, if the police barged in, his friend might go berserk. Someone could get hurt. But what choice was there?

The police pounded on the door just as Mr. Guerrero put a handful of pills into his mouth. He spent the next few weeks in a private inpatient treatment program for PTSD.