I first saw this as a history class double feature with Dr. Strangelove when I was a teen, which was certainly educational as to the contemporary view on nuclear war but deeply diminished both films as art objects in and of themselves. Divorced from that context and standing on its own, this film is pretty tremendous. It's almost universally reserved, which makes the constant steady buildup of emotion almost invisible until it really hits you in the final act.
The cinematography is similarly understated but affecting. Groeteschele walking through total shadow the instant he says death is preferable to communism. The room Henry Fonda and Larry Hagman are given in their scenes to just act—acting which forms the foundation of the film's crescendo of feeling. Fonda hiding his face as he gives the final order.
I'd generally consider myself pretty inured to the logics of nuclear thrillers, and I'd specifically seen this one before. Despite that, it still got me—I was weeping by the end, clasping my wife's hand.