I left the Army a long time ago, but our units and sections(military intelligence) and the combat arms units that I’ve been to were pretty sparse, other than Asians. OTOH, signal, supply, motorpool, laundry & bath, and clerical were sometimes the other way around.
I didn’t serve, but from what I understand the combat units are disproportionately white, especially as compared to the overall population aged 18-35. I would think this group is also largely from non-urban areas.
In other words, I would think most come from a conservative background that respects things like the Second Amendment. So I would just have a hard time seeing the existing military turn on the population in the service of a tyrant.