Christian Beyer worked around the ground-shaking blasts of one of the Army’s most powerful weapons — the M1 Abrams tank — for 23 years. And for nearly all that time, he was a model soldier, given awards for meritorious service and promoted all the way up to master sergeant in charge of training young tank crews.

Then in 2020, at age 38, he started to fall apart.

He couldn’t sleep. His family noticed that his balance had turned unsteady and he began to slur his speech. He would weep about small things and dwell on imagined conspiracies.

He grew mean, then dangerous. One night late in 2021, according to Army documents, he shoved his wife during an argument and then grabbed for a kitchen knife when a senior sergeant tried to calm him down.

As Sergeant Beyer’s struggles were beginning, the military was just starting to recognize that firing heavy weapons could lead to brain damage. Under a mandate from Congress, it began to set up programs to track and limit exposure, announced its first safety threshold for blast intensity to avoid “adverse brain health outcomes,” and drew up a list of 14 weapons that might pose a significant risk.