he intended on invading Poland and he didnt want the Soviet Union to interfere so he signed an agreement that he would directly rule the western part and he would have military rule in the middle and give the eastern part to SU plus a few other small countries and agrees not to invade their part or attack them. In the end (a few years later a believe) he invades and attacks them anyway, so it was basically to pacify them she he could invade Poland
