ima . ▁This ▁developed ▁into ▁a ▁bl oss oming ▁field ▁in ▁the ▁late ▁ 1 8 th ▁century ▁which ▁was ▁known ▁as ▁Rang aku ▁( D utch ▁studies ). ▁It ▁became ▁ob sole te ▁after ▁the ▁country ▁was ▁opened ▁and ▁the ▁s ak oku ▁policy ▁col lapsed . ▁There after , ▁many ▁Japanese ▁students ▁( e . g ., ▁K ik uch i ▁D airo ku ) ▁were ▁sent ▁to ▁study ▁in ▁foreign ▁countries , ▁and ▁many ▁foreign ▁employees ▁were ▁employed ▁in ▁Japan ▁( see ▁o - y ato i ▁ga ik ok uj in ). ▁ ▁The ▁policies ▁associated ▁with ▁s ak oku ▁ended ▁with ▁the ▁Convention ▁of ▁Kan ag awa ▁in ▁response ▁to ▁dem ands ▁made ▁by ▁Com mod ore ▁Perry . ▁ ▁Ch alleng es ▁to ▁se clusion ▁Many ▁isolated ▁attempts ▁to ▁end ▁Japan ' s ▁se clusion ▁were ▁made ▁by ▁expand ing ▁Western ▁powers ▁during ▁the ▁ 1 7 th , ▁ 1 8 th ▁and ▁ 1 9 th ▁centuries . ▁American , ▁Russian ▁and ▁French ▁ships ▁all ▁attempted ▁to ▁eng age ▁in ▁a ▁relationship ▁with ▁Japan ▁but ▁were ▁rejected . ▁ ▁In ▁ 1 6 4 7 ▁Portuguese ▁war ships ▁attempted ▁to ▁enter ▁Nag as aki . ▁The ▁Japanese ▁formed ▁a ▁block ade ▁of ▁almost ▁ 9 0 0 ▁boats ▁to ▁stop ▁the ▁ships . ▁After ▁the ▁event , ▁the ▁Japanese ▁added ▁more ▁security ▁to ▁Nag as aki ▁as ▁fear s ▁rose ▁that ▁other ▁countries ▁would ▁challenge ▁the ▁new ▁se clusion ▁policy ▁and ▁attempt ▁to ▁enter ▁through