5. What is the “closed country policy”? Why was it enforced?

Sakoku (??, "closed country") was the isolationist foreign policy of the Japanese Tokugawa shogunate (aka Bakufu) under which, for a period of over 220 years, relations and trade between Japan and other countries were severely limited, nearly all foreign nationals were barred from entering Japan and common Japanese. From 1633 until 1853, the military governments of Japan enforced a policy of sakoku or 'closed country' which prevented foreigners from entering Japan on penalty of death, and prohibited Japanese citizens from leaving.