It's worth pointing out that the rating with which Dunkirk has been classified is only applicable to the American ratings board, the MPAA, and not our own IFCO - but logic follows that if that MPAA gave it PG-13, IFCO will do the same.
However, there's been some speculation that Nolan's vision of World War II is somehow going to be watered down or less so, particularly in lieu of a snarky tweet from Jeff Bock, a box-office analyst who runs the popular Exhibitor Relations account.
If war is hell, I suppose Christopher Nolan's latest, DUNKIRK, is only mildly hellish: PG-13 for intense war experience and some language.— Exhibitor Relations (@ERCboxoffice) March 15, 2017
The rating issued by MPAA specifies that it is for "intense war experience and some language", which we'd more or less expect but it makes no specific reference to gore or violence. Steven Spielberg's Saving Private Ryan - which featured a sniper being brutally shot through the eye - was rating 15 by BBFC and rated R by the MPAA.
Likewise, Hacksaw Ridge was rated by R by the MPAA, 16s by IFCO and 15 by BBFC - and that had half a dead body being hoisted up and used as a human shield.
Another reason why I'm not excited for #Dunkirk... its PG-13 rating. You can't do the horror of war justice with that...— DreamOfOpinions (@DreamOfOpinions) March 15, 2017
The fact that Dunkirk is rated PG-13 just for the fact that Harry Styles is in it makes me nervous for the film to be honest— Connor Stevens (@moviemaniac191) March 15, 2017
Disheartened to hear #Dunkirk is rated PG-13. War, in real life, is R-rated and now DUNKIRK could feel less impactful.— Cinema Snopes (@CinemaSnopes) March 13, 2017
The point is that ratings aren't all that emblematic of what's really in a film. Let's look at Christopher Nolan's The Dark Knight, which featured a scene where a guy was brained with a pencil and a guy being horribly mutilated by an explosion which killed off a central character. Rating for The Dark Knight by MPAA? PG-13.
Nolan's proven again and again that he is more than capable of working with the confines of a strict system, and he managed to bring in three intense and psychologically complex tentpole blockbusters with a PG-13 rating. More to the point, there's never been a scene or sequence in any Christopher Nolan film where you'd think that he's being held back on account of a rating.
Inception, The Prestige, Interstellar - they all had PG-13 ratings, but would you honestly describe any of them as holding back? Of course not. In short, calm down, people. It's Christopher Nolan. He's got this.