Gemara

(Mishnah): A woman may not be secluded with Nochrim, for they are suspected of illicit relations;

Contradiction (Mishnah): If a woman was taken captive by Nochrim, if this was due to money owed to her captor, she is permitted to her husband. If she was sentenced to die, she is forbidden to her husband.

Resolution (Ravina): L'Chatchilah she may not be secluded with Nochrim, but b'Di'eved we are not concerned.

Rejection: Perhaps (normally) we are concerned even b'Di'eved. Here is different, for her captor fears that if he has relations with her, her husband will not pay her debt to redeem her.

Support (Seifa): If she was taken for Nefashos, she is forbidden to her husband (because her captors have nothing to lose).

This cannot be challenged.

Kesuvos 27a - Opinion #1 (Rav): An example of Nefashos is the wife of a thief.

Opinion #2 (Levi): An example is the wife of Ben Dunai (a murderer).

Opinion #1 (Chizkiyah): This applies only when there was a verdict for death.

Opinion #2 (R. Yochanan): It is even if there was no verdict.

Rishonim

Rambam (Hilchos Isurei Bi'ah 18:30): If a woman was taken due to Nefashos she is forbidden to Kehunah. Therefore, if her husband is a Kohen, she is forbidden to him unless someone testified for her, like a captive.

Magid Mishneh: The Rif and Rambam did not mention abduction of wives of thieves or murderers. It seems that they explain that the kingdom seizes them because they know the whereabouts of their husbands and help them, so the kingdom might kill them, so there was no need to mention them (they are like women abducted for their own capital offenses). Rashi and others explain that even though the wives are not in danger, since the kingdom will kill their husbands, the king is not concerned for (what people do to) their wives.

Rosh (Kesuvos 2:29): When a woman is taken for Nefashos even an Eshes Yisrael is forbidden, lest due to fear for her life she consented to them to find favor in their eyes. The Halachah follows Chizkiyah, for he was the Rebbi of R. Yochanan.

Ran (Kesuvos 11a DH Ela): Tosfos says that when she was taken due to Nefashos, even an Eshes Yisrael is forbidden. Perhaps she consented and willingly prettied herself to find favor in their eyes. However, R. Chananel, the Ge'onim and Rambam explain that our Mishnah discusses only an Eshes Kohen, but we permit an Eshes Yisrael even if she was taken for Nefashos. The Mishnayos before and after our Mishnah discuss Kehunah. This is why our Mishnah says 'a woman' (without specifying an Eshes Ish). The Seifa mentions her husband, to teach that even being taken due to her husband is considered 'for Nefashos.' Also, it teaches that Nochrim do not fear even when she has a husband.

Ritva (Kesuvos 26b DH u'Mihu): The Ro'oh says in the name of the Ramban that we permit an Eshes Yisrael even if she was taken for Nefashos. There is a Chazakah that she does not consent to Bi'ah.

Ritva (27a DH Heichi): Rav holds that the kingdom normally hangs thieves and make their wives and houses Hefker. Levi says that it does so only for murderers. In Avodah Zarah, one version forbids b'Di'eved whenever she was secluded with a Nochri who did not fear losing his money. Here Levi permits thieves' wives, and Chizkiyah permits even a murderer's wife as long as he was not sentenced! This is not difficult for Rashi, who explains that the Seifa discusses a Yisrael's wife (we are concerned that she consented to Zenus only if she willingly was secluded. Even if we say that the Seifa discusses an Eshes Kohen), we can say that we are more concerned when she willingly was secluded. If she was abducted via the kingdom, the captors fear (to rape her) lest she scream to the kingdom. They fear the kingdom more than they fear losing money.

Ritva (DH Omar): Some say that R. Yochanan and Chazakah discuss our Mishnah. Chizkiyah holds like Rav (who says that it applies even to thieves' wives), and requires a death sentence. R. Yochanan holds like Levi (that it applies only to murderers' wives), and therefore, he does not require a sentence. This is wrong. If so, the Gemara should have said that this argument depends on the previous one! Rather, Chizkiyah holds that even regarding a murderer's wife, they are not afraid until there is a sentence. R. Yochanan holds that even according to the opinion that the Mishnah discusses a thief's wife, since the kingdom makes her Hefker, even before there is a sentence, the captors are not afraid. The Halachah follows Rav against Levi, and Chizkiyah against his Talmid R. Yochanan.

Beis Yosef (EH 7 DH v'Chasvu Od): The Tur rules like Levi, for he was greater than Rav.

Yad Malachi (Klalei ha'Talmud 559): In other places that Rav argues with Levi, the Rif, Rambam, Rashi, Tosfos, the Rosh and Ran seek reasons to rule like one of them. There is no general rule whom we rule like.

Terumas ha'Deshen (92): A woman was seized for stealing. Amidst coersion she admitted, and was sentenced to die. Her community redeemed her for a great sum. Almost all forbid even an Eshes Yisrael, except for the Rambam. Even though there was no sentence, and the Rosh rules like Chizkiyah, that is only when she was taken due to her husband's offense.

Birkei Yosef (17, citing Radvaz 2318): The Ran cites Ge'onim like the Rambam, and the Ramban, Rashba, Ro'oh and Ritva agree. Their opinion is primary.

Poskim

Shulchan Aruch (EH 7:11): If a woman was taken for Nefashos, she is forbidden to Kehunah. Therefore, an Eshes Kohen is forbidden to her husband.

Rema: Some say that if she was seized for Nefashos, she is forbidden to her husband even if he is a Yisrael. We suspect lest she consented to them in order that they not kill her.

Beis Yosef (DH Ishah): Rashi says that if she was seized for Nefashos, i.e. execution, since they make her Hefker, we are concerned lest she consented to one of them.

Beis Shmuel (22): If one was seized for Kidush Hash-m, we are not concerned lest she consent to sin.

Beis Shmuel (23): The Mordechai (Kidushin 548) says that there is a Chazakah that she consented. This is difficult, for only warning and seclusion forbids! This is why the Maharik said that the Isur of a Shevuyah to a Kohen is a mere stringency of lineage. Those who forbid due to letter of the law must say that we are more stringent about Nochrim.

Rema (ibid.): This is only if she was seized due to herself, and there is concern for death. If she was seized due to her husband, she is forbidden only if there was a sentnce for execution.

Chelkas Mechokek (17): The Rema connotes that this is according to both opinions (of whether there is concern even for an Eshes Yisrael). If so, this is unlike he wrote above, that she is permitted to a Kohen only if the captor would lose his money. R. Elchanan forbids whenever the Nochri has no monetary claim against her. Surely he rules like R. Yochanan! To resolve the Rema we must say that here discusses only an Eshes Yisrael.

Beis Shmuel (24): Surely the Rema discusses only an Eshes Yisrael, like his previous words.

Rema (ibid.): Some say that whenever she expects to live through redemption, and return, she is permitted to her Yisrael husband, even if she was seized due to herself.

Chelkas Mechokek (18): The Terumas ha'Deshen said that one may not rely on this, but in any case we permit an Eshes Yisrael.

Rema (ibid): At a time of mass murder, Chachamim permitted her to her Yisrael husband like the first opinion.

Beis Shmuel (27) and Gra (30): This is when her consent for Zenus would not save her.