On New Year’s Day, a truck rammed into a crowd in New Orleans, killing at least 14 people and injuring dozens more. The suspect, identified as 42-year-old Shamsud Din Jabbar, a United States citizen and Army veteran, was apprehended and killed on the scene of the attack, after firing on law enforcement. The night before, he recorded a video message for his family informing them he had joined ISIS – whose flag he carried in the truck.

The same day, Matthew Livelsberger, an active-duty US Army Green Beret soldier, blew up a Tesla Cybertruck outside the Trump International Las Vegas hotel, injuring seven people. He was found dead inside the vehicle, a gun at his feet, having died by suicide shortly before the blast.

Though both men had served in the military and deployed to Afghanistan, the Pentagon press secretary said there was no evidence of overlap between the two incidents, which are both still under investigation by the FBI. Yet both cases indicate common problems facing the US military today.

I am an oral historian researching veterans’ experiences with violence in the War on Terror, the US-led global military response to the September 11 2001 attacks. Through my research, I’ve encountered significant connections between military service and violent extremism.

Most military personnel go on to lead peaceful, well-adjusted lives. But US military members and veterans make up around 28% of the nations’s mass shooters and only 7% of its general public. Being a military member or veteran is on par with other statistically significant backgrounds of mass shooters, such as being a perpetrator of domestic violence (27%).

The data on mass violence shows that a history of inflicting violence in one setting can normalise using it to express one’s views and maintain social order.

After attacks of mass violence, we commonly see widespread speculation that the perpetrator was mentally ill, particularly if they were a veteran. Yet most people who have a mental illness (including veterans) are nonviolent. In fact, experts of mass violence agree that mental illness tends to be incidental to attacks, rather than a motivating force behind them.

However, perpetrators of mass violence do have something else in common. They hold certain groups, or society at large, responsible for their suffering.

Farhad Khosrokhavar, a sociologist who pioneered research into radical groups in the 1970s and 1980s, argues individuals who become radicalised tend to feel humiliated in their own society and victimised by power structures around them. They also tend to feel connected to a group they perceive as under attack – either as a part of the group, or as a defender of it.

The manifestos and suicide notes of perpetrators of mass violence – from mass shooters to suicide bombers – show a deep........

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