Corrective note on Zen Buddhism, Japanese militarism, etc.
The religious orientation associated with the Japanese imperialistic expansion of 1890-1945 was called State Shinto: the native religion of Japan (reaching back before the introduction of Buddhism into the centuries BCE) developed to support nationalism, the concept of the divine emperor, and ethnic superiority. These strains were already native to Japanese culture, but were developed to a degree approaching perversion in order to support the militaristic state that developed in the first half of the 20th century CE. Buddhism had an effect on this, like it did on all of Japanese culture, but State Shinto actually involved a slight suppression of Buddhist activity. Defeat in WWII brought an end to State Shinto but remnants of its thinking still linger on, especially those that simply re-iterated ideas that were common before the Meiji modernization, such as the unique status of the imperial family, and can not be claimed as solely due to State Shinto.
Zen is one school of Buddhism, and one which had an unusually high degree of attraction to Westerners because it pares Buddhism down to essentials; but it was hardly the only important Buddhist school of thought, even in Japan. It is possible to over-estimate the role of Zen in Buddhist culture. There were Buddhist monasteries organized in Japan which were devoted to the concept of warrior monks and sometimes were the core of armies that fought with each other or with Japanese political factions, but they were not necessarily Zen. The military impact of Zen centered on the development of a warrior ethos we think of as Japanese, the samurai or Bushido, which focused on living in the current moment, having no fears of the future nor concerns with the past, and doing one's job. A similar approach to the warrior life, btw, is found in the Bhagavad Gita of Hinduism, which starts off with Arjuna balking at the idea of going into battle and killing so many other men, many of them his relatives; and Krishna explains that, essentially, this is what Arjuna was born to do, so what he needs to do is to go and fight and not bother over the moral implications.
Despite this cultural application to warriors in Japan, there is nothing very militaristic about Zen; in fact, in its Western form it became the basic philosophy of the Beatniks and their followers in the middle of the 20th century CE--hardly a rabid group of militaristic zealots.
This living in the moment, existentialist approach to life is what attracted many Westerners to Zen, and made it so popular in the 1950s-70s; although its attraction seems to have waned, and more overtly religious forms of Buddhism have become more popular, particularly the Vajrayana (Diamond Vehicle) schools based on Tantra, of which Tibetan Buddhism is the most famous but not the only example.
Thank you! I greatly appreciate a much more rational and informed approach to this.