Gemara

21b (Beraisa): The Mitzvah is to leave Ner Chanukah outside the door of one's house. At a time of danger, it suffices to leave it on one's table.

45a: They asked Rav if one may remove [on Shabbos] a Ner Chanukah lest it be seen by Chaverim (a nation amidst the Parsiyim). He permitted this.

Kesuvos 3b (Beraisa): From the time of danger and onwards, the custom was to be married on Tuesday. Chachamim did not object;

Question: If there was a decree to kill any virgin married on Wednesday, why was this a mere custom? The enactment (to marry on Wednesday) should be entirely uprooted!

Answer (Rava): They decreed that a virgin married on Wednesday must first have Bi'ah with the mayor.

Question: Why don't we uproot the enactment?

Answer: The decree is prone to be revoked. Therefore, Chachamim do not revoke their enactment due to the decree.

Sanhedrin 26a (Bnei Rachbah): At first, they said that people who gather Shemitah produce are disqualified (for testimony);

After the king demanded Arnona (a certain amount of Peros from each field), they were Machshir gatherers;

R. Yanai announced 'seed the land in Shemitah, due to Arnona'.

Gitin 17a: While Rav Yehudah, Rabah and Rabah bar bar Chanah were talking, two Chaverim took their lamp.

Rishonim

Tosfos (21b DH uv'Sha'as): The Ri says that the danger was after the Chaverim came to Bavel, like it says on 45a. How does leaving it on the table help? We find that Chaverim took the lamp from in front of Rabah bar bar Chanah [in the house]! We can say that they did not look so much in houses.

Tosfos (45a DH mi'Kamei): On the Parsiyim's holiday, they would not allow any Neros outside the house of idolatry. Why does the Gemara ask about Ner Chanukah more than about Ner Shabbos? Ner Shabbos is needed for eating, so the Parsiyim allowed it. Even though we say that at a time of danger, it suffices to leave it on his table, Rav discusses one who did not do so. Alternatively, the Ri says that "a time of danger" is not due to Chaverim, rather, a decree not to light Ner Chanukah.

Or Zaru'a (323) Nowadays that there is no danger, I do not know why people do not light in the Chatzer.

Shibolei ha'Leket (185): Since the custom was to light inside at a time of danger, this became the custom.

Tosfos (Sanhedrin 26a DH mishe'Rabu): Why did Arnona permit a Torah Isur? We must say that Shemitah is only mid'Rabanan nowadays. Alternatively, there is mortal danger if they cannot pay what the king demands. The Yerushalmi says so.

Maharil Diskin (Kuntres Acharon 34): According to the latter answer that it was Piku'ach Nefesh, what was R. Yanai's Chidush? Surely Piku'ach Nefesh overrides Shemitah! Rather, it was Piku'ach Nefesh for the poor, but not for the rich. R. Yanai enacted that everyone plant. Since it is Piku'ach Nefesh for the poor, it is permitted even for the rich.

Yisa Yosef (p. 218): Perhaps this is because nowadays Shemitah is mid'Rabanan. Likewise, we can say that there was not danger everywhere regarding Ner Chanukah, but since in some places there was danger, Chachamim enacted that everywhere it suffices to light on the table. They enacted without distinction. This would justify lighting indoors nowadays, since in some places there is danger outside.

Hagahos Ashri (Kesuvos 1:3): If Beis Din uproots a Mitzvah, there is no longer a Chiyuv to be Moser Nefesh for it. A smaller Beis Din may Mevatel an existing enactment if they assess that the situation changed and the earlier Beis Din would agree.

Kovetz Teshuvos (ha'Gaon R. Y.S. Elyashiv Shlita 1:67): The Shitah Mekubetzes says similarly. If so, why is there a Chiyuv to be Moser Nefesh for Mitzvos mid'Rabanan? We must say that the above reasoning applies to an enactment like getting married on Wednesday, which was for the benefit of the Kalah, and she could pardon it (Chelkas Mechokek 64:5).

Tosfos (Kesuvos 3b DH Takanah): Initially, the Gemara did not say that we should uproot the enactment, for the suggestion was to fix another day. Since they intend to make Yisrael transgress, they will not mind if we fix another day. This is not uprooting the decree, for there is a fixed day. However, if we cannot fix another day, for the mayor will come then, this is uprooting the enactment.

Rebuttal (Pnei Yehoshua): Initially, we did not suggest uprooting the enactment, for it is unlike a decree about shoelaces that obligates Mesiras Nefesh, like Hagahos Ashri says. Why did we answer that we do not uproot our decree? Yisre'elim will die! According to Hagahos Ashri, why must Yisre'elim die due to shoelaces? Beis Din can enact to wear a different color! Rather, also this is transgressing our law. Rather, Mesiras Nefesh is only for a Lav.

Poskim

Shulchan Aruch (OC 671:5): The Mitzvah is to leave Ner Chanukah at the opening next to Reshus ha'Rabim. At a time of danger when he may not fulfill the Mitzvah, it suffices to leave it on his table.

Taz (3): This is like Tosfos (45a, that the decree was against Ner Chanukah). Rashi says that the danger is on the Parsiyim's holiday. They would not allow any Neros other than for the idolatry. If so, what does it help to leave it on the table? Tosfos (21b) gave a weak answer, that they did not look so much in houses. According to Tosfos 45a, we can say that the Chaverim did not realize that a Ner on the table is for the Mitzvah.

Bach (8): The Tur says that there was Shmad, i.e. a decree not to observe the Mitzvah. This is unlike Rashi, who says that it was a holiday of the Parsiyim. He holds that it was not Shmad, for it applied to all nations. If so, why did Chachamim obligate lighting on the table, and enter Safek danger? The Parsiyim were adamant also about this, like it says in Gitin (17a)! Rather, it was Shmad. In such a case, one may not deviate even about shoelaces. Therefore, they did not totally uproot the Mitzvah. Enacting to light on one's table does not tup it. It does not change the Mitzvah itself. It changes only the place. This is permitted even at a time of Shmad, like we find in Kesuvos 3b that they changed the day of Nisu'in.

R. Akiva Eiger (YD 157:1): See Hagahos Ashri and the Pnei Yehoshua (above).

Divrei Yehoshua (40, cited in Kovetz Teshuvos ibid.): After the Gemara was sealed, due to danger, Beis Din of that generation enacted to light indoors. From that time, they never enacted to light outside. One who lights outside, like the Shulchan Aruch says, does not fulfill the Mitzvah at all.

Rebuttal (Kovetz Teshuvos): This cannot be. The Shulchan Aruch did not bring the Mitzvah to light outside for the days of Mashi'ach!

Divrei Yehoshua (ibid.): Initially, a virgin was married on Wednesday. From the time of danger and onwards, the custom is to marry on Tuesday. Chachamim did not protest. The Ro'oh says that even after the danger passed, virgins marry on Tuesday and Wednesday. This is even for those who are meticulous about Mitzvos.

Rebuttal (Kovetz Teshuvos): The Gemara said "we should uproot the [initial] enactment [to marry on Wednesday]!" and answered "we do not uproot our enactment due to a decree." According to Divrei Yehoshua, the enactment was uprooted! Rather, it is proper to marry on Wednesday (after the danger passed), like it says in the Rambam (Hilchos Ishus 10:15) and Shulchan Aruch (EH 64:3). However, since the custom became to marry on Tuesday, Chachamim did not protest.

Divrei Yehoshua (ibid.): We learn from the decree about virgins that when there is danger, Beis Din of that generation must totally uproot the Mitzvah. There is no need to fulfill the [original] Mitzvah unless they reinstate it.

Rebuttal (Kovetz Teshuvos): Even if there was a decree about Ner Chanukah after the Gemara, this is unlike the decree about virgins. The Rivash (373) says that one must be Moser Nefesh rather than actively transgress a Lav, but one need not be Moser Nefesh for a Mitzvas Aseh (YD 157). Ner Chanukah is no greater than a Mitzvas Aseh. There is no need to uproot it, since there is no danger, for people need not be Moser Nefesh for it!

Shulchan Aruch (673:1): One may not use Ner Chanukah even for uses of Kedushah. Some permit uses of Kedushah.

Gra (5): The Gemara connotes like this. It says that Ner Beiso has precedence over Ner Chanukah [due to Shalom Bayis]. If one may use Ner Chanukah for Kedushah, he should light one Ner for both, and put it on his table, like it says in times of danger.

Yisa Yosef: The Gra connotes that one is Yotzei on his table even not at a time of danger.