Dirk Bogarde did not receive an Oscar nomination for portraying Captain Hargreaves in King & Country.
Technically speaking, for much of the film, Hargreaves is a character who is going through the proper motions of defending Hamp. He first finds exactly what Hamp did, and then tries to tell Hamp what the Private will have to do to be able to live. At first Bogarde portrays Hargreaves as very much a man going through the motions of his job, and even in that there is a slight derisiveness in his performance and suggests at the moment that Hargreaves really does not care about Hamp's all that much. This changes though as he learns the story of Hamp's desertion and Bogarde is very effective in showing the change in Hagreaves in a subtle and rather believable fashion. It is not a major change of nature for Hargreaves, but Bogarde rather portrays it as Hargreaves recognizing that Hamp speaks the truth.
Bogarde of course gets the courtroom scenes, although this courtroom is little more than a tent above their very muddy and dingy surroundings. Bogarde excels in these scenes as he shows Hargreaves doing his very best to try to save Hamp life by pressing the issue of Hamp's condition before the desertion as well as Hamp's years of service. Bogarde approach is a bit different than say Kirk Douglas's performance in Paths of Glory, or Jack Thompson's in Breaker Morant. There is a different in Bogarde as there is less of strong moral indignation in his portrayal of Hargreaves's defense. Bogarde instead suggests more of an internal shame in Hargreaves, in that even as he gives a strong passion to Hargreaves arguments there is always a suggestion of the own guilt Hargreaves feels in the situation.
Bogarde's portrayal of the court defense is essential to where his character foes for the end of the film after it seems that Hamp will be executed. Captain Hargreaves, rather then bitterly chewing out his superior did as Colonel Dax did in Paths of Glory, is far more somber in his reaction especially because of Bogarde's approach to the part. In his scene where he confronts his superior Bogarde stays very meek and rather than stressing any hatred for the decision shows rather a feeling of guilt toward his own complicity in the fate of the doomed Hamp. It is a different take that might not have the fierce power found in Douglas's work, but Bogarde is moving in his own right by making Hargreaves part of the process rather than a hero within the film.
In terms of the two lead performances of the film Tom Courtenay does leave the strongest impression among the two with his exceptional performance, but that is not really anything against Bogarde's work. Dirk Bogarde gives a strong performance on his own even if his presence is sometimes secondary to Courtenay, but Borgarde works very well with him and their scenes together are remarkable. Bogarde's might not even be the very best performance of this type but it is a unique example of it. This film is bleak, even bleaker than similar films like Breaker Morant or Paths of Glory which both have a little hope in them somewhere, and Bogarde's take with his character adds to this bleakness by not even allow hope to glimmer from the one source it would be possible from. Bogarde's approach does not make this film an easy one to sit through but his unorthodox method is certainly an interesting one.