Here's the thing: All Quiet on the Western Front isn't actually rated.
Older movies have a reputation for being mild compared to the movies of today—but this perception of cinema's era of so-called innocence is the result of a censored chapter in history known as the Hays Code (source).
The (So-Called) Good Old Days
The Production Code of 1930—a.k.a. the Hays Code—was a set of moral guidelines Hollywood used to self-police its content from 1930 to 1968. The code contained rules against nudity, profanity, violence, ridicule of the clergy, and sexual relations between couples of different races.
In addition to being horribly offensive, it was also silly.
The code contained prohibitions against scenes depicting married couples sharing the same bed or engaging in lustful kissing—apparently couples were allowed to be in love but not actually enjoy the relationship (source).
Dodged a (Figurative) Bullet
The code was adopted the same year that All Quiet on the Western Front debuted, but it wasn't strictly enforced until a few years later, giving Milestone the chance he needed to depict the horrors of war.
And they are definitely depicted.
The violence and psychological torment suffered by its characters remain gripping more than eighty years later. In one scene, a soldier falls and grabs some barbed wire right when a bomb explodes on top of him. When the smoke clears, the soldier's severed hands remain clinging to the wire.
Yeah. We think "severed hand" deserves a PG-13 even if it is shot in grainy black-and-white.