A period known as the Meiji Restoration began in Japan in the mid-nineteenth century in Japan. It would be beyond the scope of this answer to give a full account of this period, but suffice to say that beginning in the mid-to-late–19th century, after millennia of self-imposed isolation from foreign...
A period known as the Meiji Restoration began in Japan in the mid-nineteenth century in Japan. It would be beyond the scope of this answer to give a full account of this period, but suffice to say that beginning in the mid-to-late–19th century, after millennia of self-imposed isolation from foreign entities, the nation of Japan was forcibly opened to trade and communication with other nations. This ultimately led to a decades-long project of Japan redefining itself as a "modern" imperial nation in a similar mode as Great Britain and, in particular, the United States. It was the latter who had made the initial threat of military action and forced the opening of Japan's borders.
Among other things, this reopening meant political reorganization on a national scale under the Meiji Emperor (who had been little more than a figurehead during the centuries of feudal war that preceded his "restoration"), with the addition of a parliamentary government styled after the nations of Europe; a consolidation of Japanese ethnicity to the dominant group, at the expense of hundreds of thousands of lives, languages, and cultures; a restructuring of the military to more closely resemble the United States, in terms of its internal organization and its technology; and an economic pivot towards capitalism, with an emphasis on private enterprise and foreign trade.
As you can imagine, this period was one of immense upheaval across the land, as thousands of years of culture representing countless traditions both localized and widespread were being upturned. The newly minted nation (originating at first among the warlord clan that had held dominance at the time the US Navy arrived to ring the bells of change) struggled in the first decades to subjugate the nation and implement its reforms.
Of most concern to Yukio Mishima, and indeed most of the Modernist writers of Japan, was the effect this had on traditional Japanese social roles. Many Japanese, especially those of particular classes such as the samurai, found their traditional roles made obsolete by the new systems. They found themselves having to renege on principles that had been handed down for centuries in order to thrive under the new order. On the other hand, the merchant classes, which traditionally had been held in low regard by the higher castes, were able to attain to unheard-of levels of economic prosperity and political power under the new system, although to do so it was usually necessary to abandon traditional values in favor of new ones being imported from the West.
In short, it is these cultural tensions that are reflected in the histories, personalities and vicissitudes experienced by the central characters in Spring Snow.
As far as Buddhism is concerned, Buddhist practices of various kinds had been widely adopted across Japan for many centuries, and many of their core principles had ingrained themselves within traditional Japanese culture writ large. Over the preceding centuries, several variants of Buddhism had even been elevated to the rank of official ideology by various shoguns who held power and were frequently directly supported by governments on a local and national scale.
However, another aspect of the Meiji Restoration was the restoration of the traditional Japanese folk practice of shinto to the position of dominant religious ideology. Interestingly, shinto had never been much of a unified religion before this point and had mostly consisted of a vast constellation of local folk beliefs and practices in a manner not at all comparable to the way we typically understand institutional religions today; the Meiji Restoration in a way forced shinto to be codified to a hiterto unknown extent. This of course did not eliminate Buddhism from culture, and to this day various Buddhist sects remain very popular and influential. It did, however, lead to a cultural association of Buddhism with older, more traditional forms of wisdom of Japan—despite the fact that Buddhism had originally been imported from China many centuries before.