says that when Rebbi Yochanan ben Zakai saw the daughter of Nakdimon ben
Gurion picking barley kernels from the dung of Yishmaelite's beasts, he
said, "Happy is Yisrael, that when they are doing the will of Hashem, no
nation or foreigner can overtake them, and when they are not doing the will
of Hashem, He gives them over into the hands of the lowliest of the
nations...." We see from here that when Hashem deems it necessary to punish
the Jewish people, it is a lowly nation that conquers them.
However, this seems to contradict the Gemara in Chagigah (13b). The Gemara
relates that Hashem enabled Nevuchadnezar to conquer the whole world so that
the nations would not mock the Jewish people and say that Hashem 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 Hashem makes the Jewish people
subject to a lowly nation *morally*. That nation might be great in power,
but it uncivilized and all of the other nations look down upon it. Hashem
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 themselves will be disgusted by the ways of that
nation and not learn from them.
Nevuchadnezar was the ruler of the nation of Kasdim when he conquered
Yisrael. The MAHARSHA points out that the Kasdim were indeed a nation that
was uncivilized and looked down upon by the other nations, as described in
the verse (Yeshayah 23:13, see Rashi there), and the Gemara (Sukah 52b,
which says the same of the Yishmaelim). People who would act in an uncouth
manner were referred to as "Bavliyim" (Yoma 66b).
However, when it comes to national conquest (and not just the humbling of
individual Jews, as with Nakdimon's daughter), Hashem does not deliver the
Jewish people to the hands of a militarily weak nation so that other nations
should not mock the Jewish people. Rather, Hashem gives them over to a
morally low nation which nevertheless is a nation of great military power.
(M. Kornfeld -- See also Insights to Moed Katan 18:1.)