The post-war treaties had made it impossible to make patriotic films for quite some time, and the jidaigeki genre, which served filmmakers in Japan since even before the war, had matured with the times, and by the sixties, it was being used to even greater social effect. Akira Kurosawa once said that it was pointless to make a period drama if it didn't have something to say about what was going on now, and there were few who did that to greater effect than Masaki Kobayashi.
Kobayashi spent the earliest part of his career as prisoner of war. Being drafted almost immediately after he began his apprenticeship, Kobayashi refused being promoted beyond the rank of private and considered himself a pacifist, and it wasn't until his release in 1946 that he was able to return to Shochiku Studios and assist director Keisuke Kinoshita.
It's worth mentioning Kinoshita briefly, as it is very probable to believe that he was a major influence on Kobayashi at the time. Though working at Shochiku for most of his career, Kinoshita was constantly changing throughout, giving himself new subject matters and genres, or at least new ways of approaching them. Shochiku was known for it's tales of everyday, working class people, known as shomin-geki, and though directors like Ozu and Naruse were both creating beautiful pieces that reflected the outlook of many Japanese in a more austere, ever simplifying aesthetic, I feel like Kinoshita was more interested in pushing boundaries in new ways of film-making, as well as exploring the subject of lost innocence, which would become a hallmark of his style. It is wholly possible that this is where Kobayashi found his voice, among a like-minded mentor who appreciated democracy and who never shied away from his own ideals, and who in fact, grew ever more nihilistic as the sixties approached.
With several of his own films under his best, and after already tackling the direct effects of World War II in The Thick Walled Room (Kabe Atsuki Heya), it was Kobayshi then, who would explore even further into a more pessimistic worldview, directing such masterpieces as The Human Condition (Ningen no Joken), and Harakiri, both startling criticisms of Japan at the time. The Human Condition especially, was Kobayashi's firm and unrelenting criticism of war and the macho indoctrination of Japanese males at the time. Ever the pacifist, Kobayshi was able to say more about the war with single gunshots than near any other film of that time, as in any of his films, violence never supplements morality.