▁possibility ▁that ▁each ▁nation ▁had ▁been ▁aware ▁of , ▁and ▁planned ▁for , ▁since ▁the ▁ 1 9 2 0 s . ▁The ▁relationship ▁between ▁the ▁two ▁countries ▁was ▁cord ial ▁enough ▁that ▁they ▁remained ▁trad ing ▁partners . ▁T ensions ▁did ▁not ▁seriously ▁grow ▁until ▁Japan ' s ▁invasion ▁of ▁Man ch uria ▁in ▁ 1 9 3 1 . ▁Over ▁the ▁next ▁de cade , ▁Japan ▁expanded ▁into ▁China , ▁leading ▁to ▁the ▁Second ▁S ino - J apan ese ▁War ▁in ▁ 1 9 3 7 . ▁Japan ▁spent ▁considerable ▁effort ▁trying ▁to ▁isol ate ▁China , ▁and ▁endeav ored ▁to ▁secure ▁enough ▁independent ▁resources ▁to ▁att ain ▁victory ▁on ▁the ▁main land . ▁The ▁" S ou thern ▁Operation " ▁was ▁designed ▁to ▁assist ▁these ▁efforts . ▁ ▁Starting ▁in ▁December ▁ 1 9 3 7 , ▁events ▁such ▁as ▁the ▁Japanese ▁attack ▁on ▁USS ▁Pan ay , ▁the ▁All ison ▁incident , ▁and ▁the ▁N ank ing ▁Mass acre ▁sw ung ▁Western ▁public ▁opinion ▁sharp ly ▁against ▁Japan . ▁F ear ing ▁Japanese ▁expansion , ▁the ▁United ▁States , ▁United ▁Kingdom , ▁and ▁France ▁assist ed ▁China ▁with ▁its ▁lo ans ▁for ▁war ▁supply ▁contract s . ▁ ▁In ▁ 1 9 4 0 , ▁Japan ▁inv aded ▁French ▁Ind och ina , ▁attempting ▁to ▁st ym ie ▁the ▁flow ▁of ▁supplies ▁reaching ▁China . ▁The ▁United ▁States ▁halt ed ▁ship ments ▁of ▁air plan es , ▁parts , ▁machine ▁tools , ▁and ▁av iation ▁gas oline ▁to