Complex sentence: To prevent further foreign ideas from sowing dissent the Tokugawa shogunate adopted the "sakoku" ("closed country") isolationist policy under which Japanese people were not allowed to travel abroad, return from overseas, or build ocean-going vessels.
Split into simpler sentences: To prevent further such foreign ideas from sowing dissent in the minds of the Japanese, the Tokugawa shogunate adopted a foreign policy of isolationism known as "sakoku" ("closed country"). Under this policy, no Japanese people were allowed to travel abroad, to return from overseas, or to build ocean-going vessels.