This is really an outstanding movie which is not a "war movie" but rather a complex psychological one of what drive us as a person to take a stand when confronted to situations we could not have ever imagined. Of course education, training, the society, family history, the culture of the country in which you were born, all of this will play a major role with regard to the choice(s) that we will take in life. However, as per prior personal experiences and call it maybe "the genetics" these choices will be different for people even having the same cultural background. The life story of lieutenant Onoda is maybe what you could call an "extreme" one which, besides cultural background, was certainly facilitated by his "physical adaptation" allowing him to endure the certain hardships and harrowing experiences he encountered during his many years of isolation.
Onoda is a long movie but its length was necessary not to develop the psychological development of the main character but rather to allow the viewer to "feel" what it must have meant in terms of hardship, endurance and faith the choices that Lieutenant Onoda took to honor his pledge of obedience regardless the validity of the cause he chose to defend.
The fact the movie Onoda was released in full pandemic, of the relatively unknown nature of the real history of lieutenant Onoda and that the film must only be shown in the Japanese language (with subtitles) as an indispensable component of the story will certainly limit its diffusion in theaters. Still this will not hamper the fact that this is cinema of the highest caliber and that Arthur Harari, its French director succeeded in making an admirable movie not to be missed!