The War Game (1965) dir Peter Watkins

A prototype to Threads/ The Day After, The War Game invented the docudrama form and examines hypothetical fallout from a nuclear attack on Britain in horrifying footage.

As an aside, curious if any one knows if this film is the origin of the record label name Honest Jon's. In the film, a nuke is nicknamed an Honest Jon and I haven't seen any other colloquial use of the term.