sentence: on september 13 , a franco - british task force landed in java , main island of the dutch east indies ( for which independence was being sought by sukarno ) , and saigon , capital of cochinchina ( southern part of french indochina ) , both being occupied by the japanese and ruled by field marshal hisaichi terauchi , commander-in-chief of japan 's southern expeditionary army group based in saigon .