Why did Japan close itself off from the world for two centuries?

Japan closed itself off, the standard story goes, because the Tokugawa shogunate feared that Spain and Portugal, and the Catholic religion they brought, threatened its stability and peace. The policy, called sakoku, severely limited relations and trade with other countries, banned almost all foreigners from entering and kept ordinary Japanese from leaving. It was enacted under the shogun Tokugawa Iemitsu through edicts issued between 1633 and 1639.

But the story is not that simple, and some scholars, building on work by Japanese historians in the 1970s, call it only a partial explanation. A second motive was control at home. The Tokugawa feared that daimyo who traded freely abroad could grow strong enough to challenge them, so by restricting foreign trade they could make sure none became too powerful. The direct trigger is said to be the Shimabara Rebellion of 1637 to 1638, an uprising of 40,000 mostly Christian peasants. Afterwards the shogunate expelled missionaries and banned Christianity on penalty of death.

The surprise is how open the "closed country" was. Japan traded with five entities through four "gateways". The only European influence allowed was a Dutch factory at Dejima in Nagasaki, and through it Western science and medicine flowed into Japan as "Dutch learning". Trade prospered, and the country was far from closed. In fact the very word sakoku was coined only in 1801, by the astronomer and translator Shizuki Tadao. Because it implies a country that was totally closed, many scholars now prefer the term of the time, kaikin, meaning "maritime prohibitions".

So the doors were guarded, not bolted, until foreign ships began to push.