I’ve heard on several occasions that the holocaust was predicted in the Talmud in Megillah 6b. These claims seemed to have emerged post-holocaust, however. So, my question is, prior to the holocaust, what was the traditional understanding of the meforshim regarding the passage in Megillah 6b? Could you cite some sources.

Our sages teach us in several places that preceding the redemption may very well befall us national calamities. For example, the parable in Midrash Tehilim (20), "When you see the cemetery, know that the State is soon to come". You are referring to the Talmud in Megilla 6b, which warns us specifically of "Germamia" of Europe on the verse in Tehilim 140, 9, "O Lord, do not grant the desires of the wicked; do not let his thoughts succeed, for they are constantly arrogant", on which the Talmud explains, "Lord of the Universe, don't allow Esav the Evil his desire… this refers to Germamia of Edom (term commonly used for Christian Europe), for if they go out, they will destroy the entire world". It then continues to say that they unite 300 colonies (which is what comprised the 3rd Reich!). R. Ya'akov Emdin (d. 1776/5536) in his notes there (found in the back of the classic edition of the Talmud) edits the obvious correction that it should read: Germania, Hebrew for Germany. I've seen this amendment also in the name of the Vilna Gaon (from about the same period, about 200 years before the Holocaust). Although the older manuscripts of the Talmud also read: Germamia, it should be noted that there was no nation named: Germamia, only Germania (literally: fertile land), whose name was already in use at the time of the writing of the Talmud, so the correction of the aforementioned rabbis is logical and obvious (as confirmed in the footnotes of the Dikdukei Sofrim on the ancient manuscripts, who stress that it's referring to Germany).