In one of Moshe's final messages to the people, Moshe prophesied that after his passing the people of Israel would turn away from Hashem. They will suffer greatly-going into exile and feeling as if Hashem has left them.
There have been times throughout history when this הסתרת פנים, hiding of Hashem's face, appears to have occurred. Many expulsions and massacres have cost the lives of many Jews, often in societies where Jews at one point felt secure. Certainly the Jews of Spain and Germany had every reason to believe that they had contributed greatly to the welfare of those countries. Perhaps some of them felt to secure in those lands, too willing to adapt to the gentile way of life.
Of the two ways Jews have suffered in exile, assimilation and oppression, assimilation has been the greater threat. It creeps upon us without our realizing it, and Hashem doesn't intervene in the way people live their religious lives. With physical persecution, on the other hand, Hashem can work "behind the scenes" to ease the situation, as He did with the hidden miracle of Purim.