Rebbe,
The Gemara (daf 14) indicates that Yirmiyah Hanavi went to return the 12 tribes and was successful. This leads to the obvious question - where are they? We don't find them going to Bavel with Yehuda and Binyamin. We have a tradition that they are the lost 12 tribes. There are many midrashim as to where they went and questions in Gemaras if they're coming back.
Is there another Gemara that says that they returned to Assyria after Yirmiyahu returned them to Eretz Yisrael?
Sam Kosofsky
The Ya'avetz lists a number of additional proofs from Tanach that the ten tribes integrated into Klal Yisrael. The Gemara too, talks about the application of the Yovel from then until the Churban, indicating that the ten tribes remained under the jurisdiction of the kings of Yehudah right up to the time of the Churban. Presumably, they went into Galus together with Yehudah and Binyamin (What makes you assume that that they did not?).
To reconcile this Gemara with the Mishnah in Sanhedrin (110b), which cites a Machlokes as to whether the ten tribes will return to Eretz Yisrael in due course or not, Rashi (there) comments that in fact, Yirmiyahu only returned some of the ten tribes (i.e. a few of each tribe to their original inheritance), but not all of them. Seeing as the Gemara in Erchin says that all of Yisrael need to be on their land for Yovel to be practiced, I wonder whether the few referred to by Rashi does not really mean the majority of each tribe.
What emerges then, is that the ten tribes are hidden in some place (behind the River Sambatyton), as per tradition. Yet the Golus in which we are currently living is not devoid of them (a spattering at least). The common assumption therefore, that we all descend from the tribes of Yehudah, Binyamin and Levi appears to be no more than a myth.
Be'Virchas Kol Tuv
Eliezer Chrysler