(Background: In Siman 1 the Rebbe posed a basic question: How can Rav Yehuda, an Amora, disagree with the Mishnah? Rav Yehuda says that it is forbidden even for an individual Jew to go to Eretz Yisroel nowadays, while the Mishnah says that on the contrary, going to Eretz Yisroel is such a mitzvah that either spouse can force the other to do it. Now we are ready to answer this question.)
Let us return to our subject. Now there is no contradiction at all between Rav Yehuda and the Mishnah and Baraisa about forcing one's spouse, certainly according to Rabbi Meir of Rothenberg, and even according to those who disagree with him. According to Rabbi Meir of Rothenberg, the Mishnah applies only in Temple times, while Rav Yehuda's law applies only during exile. And even according to the Tur, who disagrees with Rabbi Meir, it is still possible that the Mishnah applied only in Temple times. The Tur's problem with Rabbi Meir is only that if the Yerushalmi (which says that only the husband can force the wife, not vice versa) is talking about during exile and the Mishnah applied only in Temple times, then why should the husband have the right to force his wife? No one should be able to force anyone. So it is clear that if the Yerushalmi and Rabbi Meir of Rothenberg had simply stated that no one can force anyone nowadays, the Tur would have had no problem with the idea of confining the Mishnah to Temple times. And this is precisely Rav Yehuda's opinion.
And even according to the Chasam Sofer, who proves from the law of the slave that the Babylonian Talmud disagrees with this Yerushalmi, and argues that even Rabbi Meir of Rothenberg did not mean his distinction between husband and wife as practical halacha, only as an explanation of the Yerushalmi, there would be nothing wrong with saying that Rav Yehuda agrees with the Yerushalmi. Of course, we have already noted that language of the Rosh and Rabbi Meir of Rothenberg indicates that they are saying practical halacha, contrary to the Chasam Sofer's interpretation. The Beis Shmuel indeed rules in accordance with Rabbi Meir, and we have already answered all the questions on this from the slave and other places.
We have also resolved the problem with Rav Yehuda raised by the Rif on Ein Yaakov (Rabbi Yoshiahu Pinto): how can Rav Yehuda forbid Jews to go to Eretz Yisroel if living outside Eretz Yisroel is tantamount to idol worship? The answer was that that was true only in Temple times (because leaving a Jewish country and moving to an idolatrous country is like worshipping idols, but nowadays you are under idol worshippers no matter where you live), and even if one disagrees with this assertion, Rav Yehuda can certainly hold that way.
However, although Rav Yehuda’s statement is now understandable, it would seem that the halacha follows Rabbi Zeira. The proof to this is that the Rif, Rosh, Tur and Shulchan Aruch all codify the law of forcing one’s spouse even in our times, and Rabbi Meir of Rothenberg holds that at least the husband can force the wife, so it is clear that they didn't hold like Rav Yehuda, who says the law does not apply in our times at all. We also see in the Gemara many Amoraim who went from Babylonia to Eretz Yisroel, which shows that they held like Rabbi Zeira, not Rav Yehuda.
(This was the end of Siman 9 in the 1959 edition of Vayoel Moshe. In 1961, when the sefer was reprinted, the Rebbe added the following paragraphs:)
After the above was printed, I heard some people casting doubts on what I wrote that the Halacha is like Rabbi Zeira, because the Kesef Mishneh says that the Rambam did indeed rule like Rav Yehuda. The Rambam writes (Hilchos Melachim 5:8): “Just as it is forbidden to move from Eretz Yisroel to Chutz Laaaretz, so too it is forbidden to move from Babylonia to other lands, as the verse says, “To Babylonia they shall be brought and there they shall stay.” We see here, says the Kesef Mishneh, that the Rambam considers the two statements of Rav Yehuda (on page 110b and 111a) to be one and the same concept. On 110b he says that anyone who goes from Babylonia to Eretz Yisroel transgresses a positive command, and on 111a he says in the name of his teacher Shmuel that it is forbidden to move from Babylonia to other lands. The Rambam copies the second statement of Rav Yehuda in his code, yet he cites the Biblical verse used in the first statement. So it is clear that the Rambam understood that in the first statement, the real problem is leaving Babylonia, not going to Eretz Yisroel. Accordingly, it would be allowed to emigrate to Eretz Yisroel from any other location besides Babylon. In the second statement of Rav Yehuda (111a), when he says it is forbidden to go from Babylonia to other lands, this includes Eretz Yisroel too. And we see in the Gemara, adds the Kesef Mishneh, that many later Amoraim concurred with Rav Yehuda's second statement (this probably refers to Rabah, Rav Yosef and Abaye, quoted in the Gemara right after Rav Yehuda).
Now, the Lechem Mishneh quotes Rashi, who says that the reason for Rav Yehuda’s second statement (that it is forbidden to leave Babylonia) is because there are yeshivos teaching Torah all the time in Babylonia. If so, he clearly disagrees with the Rambam’s assertion that Rav Yehuda's second statement is based on the verse "to Babylonia they shall be brought.” According to Rashi, this second statement of Rav Yehuda in the name of Shmuel has no connection to exile, and indeed one would be allowed to go to Eretz Yisroel, since there are yeshivos there as well. Rav Yehuda's first statement, on the other hand, is based on the verse "to Babylonia they shall be brought" and is an exilic law - it forbid Jews from going back on their own to Eretz Yisroel from anywhere, "until the day that I revisit you" as the verse continues. This teaching would theoretically permit Jews to leave Babylonia and go anywhere else besides Eretz Yisroel, were it not for the second statement with its emphasis on staying in a place of many yeshivos.
The Lechem Mishneh brings proof to Rashi's interpretation, and argues that if the Rambam were correct that the two statements are making the same point, Rav Yehuda should have said, "Whoever leaves Babylonia transgresses a positive commandment," not "Whoever goes up from Babylonia to Eretz Yisroel transgresses a positive commandment." His emphasis on the words "to Eretz Yisroel" strongly indicates that it is only forbidden to go to Eretz Yisroel, not to other countries. One might argue that "to Eretz Yisroel" really means "even to Eretz Yisroel" despite its holiness, but certainly other countries are included in the prohibition. But then, argues the Lechem Mishneh, Rav Yehuda would have had no need to make his second statement about leaving Babylonia at all. The Lechem Mishneh, after a lengthy analysis, concludes that the text strongly points to Rashi's interpretation, not the Rambam's.
In any case, we see that the Lechem Mishneh agreed with the Kesef Mishneh's assessment of the Rambam's position, so that we have here a clear dispute between Rashi and the Rambam as to whether, according to halacha, it is forbidden to travel from Babylonia to Eretz Yisroel.
The Maharit (Rabbi Yosef Di Trani, lived in Greece 1538–1639) also understood the Rambam this way, and, like the Lechem Mishneh, argued that the text does not support the Rambam. The words of Rav Yehuda in the name of Shmuel, "Just as it is forbidden to move from Eretz Yisroel to Chutz Laaaretz, so too it is forbidden to move from Babylonia to other lands," strongly imply that going from Babylonia to Eretz Yisroel is permitted - in contrast with Rav Yehuda's own personal view. So how can the Rambam quote this statement and then say that it is based on the verse "to Babylonia they will be brought" - a prohibition specifically on going to Eretz Yisroel? The Maharit goes on to ask other questions on the Rambam, and leaves them unresolved.
However, there are several Acharonim (later commentators) who interpret the Rambam to mean only the second statement of Rav Yehuda, that it is forbidden to leave Babylonia to go to any land except Eretz Yisroel.
1. Rabbi Avraham Abish of Frankfurt am Main (1700's) in his commentary Emek Hamelech, printed in the back of the standard Rambam, takes issue with the Kesef Mishneh and argues that if the Rambam had meant to go so far as to forbid moving from Babylonia to Eretz Yisroel, he would have said so explicitly. Rather, it is clear that we do not rule in accordance with the first statement of Rav Yehuda, and this is borne out by many stories in the Gemara. It is only his second statement, in the name of Shmuel, that many Amoraim concur with, and this is the Rambam's opinion. However, Rabbi Avraham is still puzzled over the Lechem Mishneh's strong question: why then did the Rambam cite the verse used in the first statement of Rav Yehuda? He leaves this unresolved.
2. Rabbi Chaim ben Yaakov Palaggi (1800s, chief rabbi of Smyrna, Turkey), in his work Nishmas Kol Chai chapter 49, rules in accordance with Rabbi Zeira, not Rav Yehuda, and adds: "Even the Rambam, who rules in the fifth chapter of Melachim that it is forbidden to move from Babylonia to other lands, does not include Eretz Yisroel in this prohibition. The evidence to this is that we find many Amoraim who moved from Babylonia to Eretz Yisroel and were not concerned about Rav Yehuda's opinion. And although our master [Rabbi Yosef Karo] in the Kesef Mishneh writes that the Rambam did forbid moving to Eretz Yisroel, with all apologies, it seems that he was mistaken. For if the Rambam had meant this, he should have written explicitly that whoever goes up from Babylonia to Eretz Yisroel transgresses a positive commandment."
3. Rabbi Yaakov Emden, in the introduction to his edition of the prayerbook, rules against Rav Yehuda, although his language there is difficult to understand. (In this passage, found on page 13 of the Siddur Beis Yaakov, Rabbi Yaakov Emden criticizes Jews for forgetting Eretz Yisroel and getting too comfortable in exile. He blames the Spanish expulsion on this sin, and laments that only one or two in a thousand Jews go to Eretz Yisroel. Probably the Satmar Rebbe's comment "difficult to understand" means that it is one thing to rule that a Jew is permitted to go to Eretz Yisroel; it is quite another to imply that everyone should ideally do so.) He quotes many stories in the Talmud of countless Amoraim who moved from Babylonia to Eretz Yisroel; clearly, they all held like Rabbi Zeira. This is an irrefutable proof, for the Talmud never mentions in any of these places that there was anything legally questionable about what these Amoraim did.
But if we are to say that the Rambam agrees that the halacha is like Rabbi Zeira, we have to resolve the question of why the Rambam brings the verse, "To Babylonia they shall be brought," which forbids going to Eretz Yisroel. This was the question asked by the Maharit and the other commentators. And this seems to be what convinced the Kesef Mishneh that the Rambam was indeed ruling in accordance with Rav Yehuda.
The Iyun Yaakov, written by Rabbi Yaakov Reischer (Austria, early 1700's), author of Responsa Shvus Yaakov, provides a way to resolve this problem. The Iyun Yaakov proposes that both statements of Rav Yehuda are derived from the same verse - one from the first half and the other from the second half. "To Babylonia they shall be brought" teaches that it's forbidden to go from Babylonia to Eretz Yisroel (or from anywhere to Eretz Yisroel, for that matter, since the main issue is that one may not return from exile). I might still think that one is allowed to go from Babylonia to other lands, but then the verse continues with the seemingly extra words, "And there they will stay" - and this is the source for Rav Yehuda's second statement that one may not go from Babylonia to any other land.
With this in mind, we can explain the Rambam according to those who disagree with the Kesef Mishneh. The Rambam rules like Rav Yehuda's second statement, not his first, because it is on this point that other Amoraim agree with him (namely, Rabah, Rav Yosef and Abaye). So it is allowed to leave Babylonia and go to Eretz Yisroel, just not to other lands. And on this the Rambam cites the verse that Rav Yehuda used in support of his first statement, but the Rambam actually only meant the second half of the verse.
As far as the practical halacha, even if we accept the Kesef Mishneh's interpretation (according to which the Rambam forbids going from Babylonia to Eretz Yisroel), we would not rule like the Rambam, but rather like Rashi. The reason is that there is a rule, laid down by the Beis Yosef in Orach Chaim 159 in the name of the Terumas Hadeshen, that whenever there is a dispute between two poskim in which one expresses his opinion explicitly and it can be inferred or deduced the the other disagrees, we rule in accordance with the explicit posek. In this case, Rashi says explicitly that the second statement of Rav Yehuda has nothing to do with the first: one must stay in Babylonia because it is a place of Torah academies. The Rambam does not explicitly connect the two statements of Rav Yehuda; it is only the Kesef Mishneh who infers that from the fact that the Rambam cited the same verse.
Besides, many great Acharonim did not agree with the Kesef Mishneh's interpretation of this Rambam at all. They say that the Rambam also ruled like Rabbi Zeira, based on the Rambam's language and based on the undeniable fact that many Amoraim went to Eretz Yisroel. So this is why I wrote that the halacha is like Rabbi Zeira.