QUESTION: The Gemara says that when Rebbi Yochanan ben Zakai saw the daughter of Nakdimon ben Gurion picking barley kernels from the dung of a Yishmaelite's beast, he said, "Happy is Yisrael, that when they are doing the will of Hash-m, no nation or foreigner can overtake them, and when they are not doing the will of Hash-m, He gives them over into the hands of the lowliest of the nations...." When Hash-m deems it necessary to punish the Jewish people, it is a lowly nation that conquers them.

The Gemara here seems to contradict the Gemara in Chagigah (13b). The Gemara relates that Hash-m enabled Nevuchadnetzar to conquer the whole world so that the nations would not mock the Jewish people and say that Hash-m gave His people over to a lowly nation. A similar statement appears in Gitin (56b), "Whoever oppresses Yisrael becomes the head [of a nation]."

ANSWER: The Gemara here in Kesuvos means that Hash-m makes the Jewish people subject to a nation which is morally corrupt. That nation might be great in power, but it is uncivilized and all of the other nations look down upon it. Hash-m does not want the Jewish people to learn the ways of the nation that conquers them, and therefore He makes a morally low nation conquer them so that the Jewish people will be disgusted by the ways of that nation and not learn from them.

Nevuchadnetzar was the ruler of the nation of Kasdim when he conquered Yisrael. The MAHARSHA points out that the people of Kasdim were uncivilized and looked down upon by the other nations, as the verse (Yeshayah 23:13; see Rashi there) and the Gemara (Sukah 52b) describe (the Gemara there says the same about the Yishmaelim). People who acted in an uncouth manner were referred to as "Bavliyim" (Yoma 66b).

When Hash-m deems it necessary to cause a national conquest (and not merely the humbling of individual Jews, as was the case with the daughter of Nakdimon), He does not deliver the Jewish people into the hands of a militarily weak nation, lest the other nations mock the Jewish people. Hash-m gives them over to a nation of great military might, but which is morally corrupt. (M. Kornfeld) (See Insights to Moed Katan 18:1.)