When my daughter’s MMA boyfriend sent her to the ER, I drove directly to his gym. His coach eyed my gray beard and laughed, “Get out of here, Grandpa, unless you want to leave in a body bag.”

For fifteen years, I taught Marines how to turn their bare hands into lethal weapons. As a Force Recon hand-to-hand combat instructor at Quantico, I trained MARSOC Raiders and over three thousand combat-ready troops. I lived by a single absolute…