Sgt. 1st Class Matthew Bessler describes some of his military experience outside his home in Powell, Wyo., in 2015. (Bonnie Jo Mount/The Washington Post)
Dillon Carroll is a historian and freelance writer in Sacramento, and is currently writing a book on Civil War veterans and mental illness.

This afternoon, President Trump will award Garlin Murl Conner, a first lieutenant in World War II, a posthumous Medal of Honor, the nation’s highest military honor. In addition to discussing his achievements, Pauline Conner spoke to the media about her late husband’s struggles. “I’ve always said if anybody ever had PTSD, he did,” she recounted. “Because many of the time, he’d wake up in the night with nightmares, and after I would wake him up, he would go outside, sit on the porch and smoke cigarettes for hours at a time. But he still wouldn’t talk about what was happening.”

Conner’s frank comments about her late husband’s post-traumatic stress disorder highlight the enduring effect violence can have on the human brain. Our understanding of these effects, however, went either unknown or denied until relatively recently. Psychologists only began designating PTSD as a defined disorder in 1980. But the condition, and our understanding of it, has evolved over more than a century, an evolution that cannot be understood outside of the context in which it emerged: the violence of the 20th century.

PTSD is a cluster of symptoms that plague some people in the wake of a traumatic experience such as accidents, natural disasters, rape, assault and war. Victims of PTSD often experience symptoms like hypervigilance and hyper-arousal, emotional numbing, as well as intrusive nightmares, flashbacks and hallucinations of the traumatic event.

There is sporadic anecdotal evidence that ancient and not-so-ancient warriors suffered from the aftereffects of war. Herodotus, a Greek soldier, recalled an Athenian who went blind from fear during the Battle of Marathon. Thousands of Civil War veterans were admitted to insane asylums. But neither medical professionals nor the public had any firm conviction that trauma could have sustained psychological consequences.

World War I changed that. The conflict exposed what trauma could do to the nerves and the mind. Because of the sheer number of combatants — millions of men shouldered a rifle and stained the grass of Flanders Field and the splashing waters of the Somme with their blood — the numbers of emotional and psychiatric casualties were too vast to be ignored.

Doctors developed several diagnoses to classify the intense feelings of fear and helplessness that sometime led to symptoms of psychiatric impairment. “Shell shock” was the most evocative diagnosis, reflecting the new killing machines of the war. Doctors believed that concussive shocks of artillery explosions could damage the nerves, with crippling psychological consequences.

Once home, many veterans relived the war through nightmares, just as Garlin Conner did. As British soldier Siegfried Sassoon wrote while convalescing in a hospital: “And when the lights are out … then the horrors come creeping across the floor: the floor is littered with parcels of dead flesh and bones. Yet I found no bloodstains there this morning.” Robert Graves recalled when he returned home he “was still mentally and nervously organized for War. Shells used to come bursting on my bed at midnight, even though Nancy shared it with me; strangers in the daytime would assume the faces of friends who had been killed.”

The servicemen who marched off to war in Europe and the Pacific during World War II suffered through experiences that produced similar feelings of fear and helplessness. Instead of shell shock, psychiatric trauma during World War II was frequently diagnosed as “combat exhaustion” or “combat fatigue.” During the war, the U.S. Army recorded three times as many psychological breakdowns as it had in the First World War. More than 300,000 men were discharged with psychiatric symptoms — 43 percent of all men discharged for medical reasons.

The psychological effects of World War II on the “greatest generation” remain shrouded in secrecy, because, as poet Karl Shapiro wrote, it was also a “generation of silence.” Americans who volunteered went to war on the heels of the Great Depression. They came from a social and cultural landscape that stressed stoicism and uncomplaining fortitude. As long as they could generally function in society, it was believed that they were recovered. So the servicemen who came home from Berlin or Tokyo stayed quiet, and the emotional toll of the war remained secret.

But their stories did not go entirely untold: Postwar culture reflected the vast psychological toll of the war. William Wyler’s 1946 film “The Best Years of Our Lives” frankly portrayed returning veterans who were struggling with alcoholism, marital problems and nightmares of combat. John Huston’s stunning film “Let There Be Light,” chronicled veterans suffering with nervous conditions recovering in a military psychiatric facility. The movie was considered so controversial by the Army that it suppressed production of the film for several decades.

Kurt Vonnegut’s “Slaughterhouse-Five” became the consummate PTSD novel, even though it was published more than a decade before the condition was named. Billy Pilgrim, the protagonist, becomes literally unstuck in time, going from his near-death experience at the bombing of Dresden, to his life back in the United States with no rhyme or reason. Clearly, enough veterans were haunted by the war that it seeped through in books and film.

After years of discussions about “shell shock” and “combat fatigue,” the Vietnam War provided the catalyst for creating the diagnosis of PTSD as a specific disorder. American soldiers came home with what psychiatrists began calling “delayed psychiatric trauma” and “post-Vietnam syndrome.” Symptoms often emerged months or even years after a soldier’s tour of duty, and often included a shared set of symptoms: nervousness, anger, excessive emotional reactions, sleeplessness, intense guilt and shame and intrusive flashbacks and nightmares.

In Vietnam, veterans’ traumatic experiences in the conflict were aggravated by their sense of exploitation by the government, isolation from civil society and lack of victory. In 1970 Jan Barry, a member of the group Vietnam Veterans Against the War wrote to psychiatrist Robert Lifton asking for his help. Lifton, who began holding informal “rap sessions” in the group’s headquarters in Manhattan, increasingly felt this was a new and singular disorder that deserved recognition.

To secure that recognition, Lifton and others began lobbying the psychiatrists in charge of revising the “Diagnostic and Statistical Manual of Mental Disorders” (commonly known as the DSM), a collection of more than 200 classified mental disorders. In 1980, the third edition of the DSM was published and with it a new diagnosis: PTSD.

Naming PTSD had significant consequences. It created clear criteria for diagnosis and helped establish new treatment regimes. It also enabled doctors and therapists to diagnose the effects of trauma in a population beyond veterans: Today, women are twice as likely as men to be diagnosed with PTSD, which is most commonly caused by sexual violence rather than war. Currently, it is estimated that 10 to 12 percent of combat veterans have diagnosable PTSD, compared to 50 percent of rape victims.

The evolution of PTSD reflects our changing understanding of trauma and its effects on the mind. The diagnosis and our understanding of it are part of a patchwork of the new and the old, a blend both of what we have known and what we are learning. And it reminds us that war is not only destructive work, but self-destructive work.