Artists interpretation of Amakusa Shiro, leader of the Shimabara Rebellion.
From Wikipedia
The Shimabara Rebellion (島原の乱 Shimabara no ran) was an uprising in what is now Nagasaki Prefecture in southwestern Japan lasting from December 17, 1637, to April 15, 1638, during the Edo period. It largely involved peasants, most of them Catholic Christians.
It was one of only a handful of instances of serious unrest during the relatively peaceful period of the Tokugawa shogunate's rule.[2] In the wake of the Matsukura clan's construction of a new castle at Shimabara, taxes were drastically raised, which provoked anger from local peasants and rōnin (samurai without masters). Religious persecution of the local Catholics exacerbated the discontent, which turned into open revolt in 1637. The Tokugawa Shogunate sent a force of over 125,000 troops to suppress the rebels and, after a lengthy siege against the rebels at Hara Castle, defeated them.
In the wake of the rebellion, the Catholic rebel leader Amakusa Shirō was beheaded and the prohibition of Christianity was strictly enforced. Japan's national seclusion policy was tightened and official persecution of Christianity continued until the 1850s. Following the successful suppression of the rebellion, the daimyō of Shimabara, Matsukura Katsuie, was beheaded for misruling, becoming the only daimyō to be beheaded during the Edo period.
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Christianity has never really taken root in Japan the way it did in their neighbors Korea, the Philippines, and even China. This was because the Samurai, the ruling class of the era were very suspicious of foreign influence. Which led to the creation of a government policy that demanded an expulsion of all Europeans from Japan, this also extended to those the Europeans had converted to their faith. Japanese Christians.
This was done because Japan heard news of Christianity becoming a 'third column' of sorts, used by European colonists to get a portion of the native population on their side, and then turning the New Converts against the natives who preferred to stay with the way of their ancestors. (See the Phillipines)
The Christians held out for a good long time in the fortress. Only ultimately losing because of a deal the Daimyo and the Shogun made with the dutch for exclusive trade rights with Japan.
The leader of the rebellion, Amakusa Shiro was counting on the Europeans to help him with the fight. But those same Europeans kind of sold him and every Christian on shimabara out.
The shogunate made a deal with the Dutch merchants to keep exclusive trade rights with Japan during the era of isolation in exchange for Lending them cannons to help bring down the barriers keeping them from taking the castle at Shimabara.
What if, in another timeline. The Dutch had sided with the Martyrs Of Shimabara?