Which kingdom did the samurai serve?

Japan, though serving is doing a lot of work in that sentence - the samurai were retainers of regional lords, the daimyo, and for most of their history the emperor was a ceremonial figure while real power sat with a shogun. Loyalty ran up a chain that frequently did not reach the throne.

The class was also abolished by the government it had created. The Meiji Restoration of 1868 was carried out largely by samurai from outlying domains, who then dismantled their own class - ending the stipends, banning the wearing of swords in public, and creating a conscript army of commoners, on the reasoning that a modern state could not afford a hereditary warrior caste. The Satsuma Rebellion of 1877 was the last armed protest against that, and it was crushed by the conscripts.

Bushido, the code everyone cites, is mostly a later codification. The formal literature on it was written during the Tokugawa peace, when samurai had become administrators with nothing to fight, and the version most widely read in English is Nitobe Inazo's book from 1900 - written in English, for a Western audience, by a Christian convert who had not lived in a samurai world. It was then adopted enthusiastically by the militarists of the 1930s, which is how a nostalgic Victorian text became a wartime ideology.