Gemara

(Beraisa - R. Shimon): A woman is divorced willingly or unwillingly, but a man divorces only willingly.

One might have thought that Chachamim enacted that a woman must pay a Kesuvah when she (vexed him, and) forced the divorce. R. Shimon teaches that a man is never forced to divorce. He can delay giving a Get.

88b (Mishnah): A Get coerced by (a Beis Din of) Yisre'elim is valid.

Rav Yosef was forcing a man to give a Get.

Question (Abaye): We (in Bavel) are like commoners. We cannot coerce Gitin!

(Beraisa - R. Meir): "These are the judgments that you will put in front of them" - one may not go for judgment in front of Nochrim, even if their law (in the case) is the same as Torah law;

Also, we expound "before them (Mumchim, i.e. expert judges)", and not before commoners.

Answer (Rav Yosef): We are Sheluchim (to coerce Gitin) on behalf of the Mumchim in Eretz Yisrael, just like we are their Sheluchim to judge cases of admissions (of debt) and loans.

Bava Basra 47b (Rav Huna): If Reuven hung (or afflicted in another way) Shimon until Shimon agreed to sell his property, the sale is valid.

Support: In every sale, the owner is coerced. He would not sell unless his need for money, and nevertheless it is valid.

Objection: Perhaps a self-induced Ones is different than an externally induced Ones!

48a: Amidst the coercion, he decides absolutely to sell.

Support (Beraisa): (If one is obligated to divorce his wife but refuses to,) we force him until he says 'I want' (to authorize Sheluchim to write and give a Get).

Objection: Perhaps there he truly wants, because it is a Mitzvah to heed Chachamim (who ruled that he must divorce her)!

Rishonim

Rif and Rosh (9:10): A Get coerced legally by Yisre'elim is valid. Even though in Bavel there are no Mumchim, we are Sheluchim (to coerce) of Mumchim in Eretz Yisrael, just like we are their Sheluchim to judge cases of admissions and loans.

Ran (DH Anan): Presumably, even when there are no Mumchim in Eretz Yisrael, we are Sheluchim of Mumchim in previous generations. Judges in Bavel were Machshir converts based on assumptions that they immersed (without explicit authorization for this)! Rather, we are Sheluchim whenever this is needed. We are Sheluchim for loans and admissions, lest people be loathe to lend. Likewise, we are Sheluchim to coerce Gitin, lest women be unable to remarry!

Rambam (Hilchos Gerushin 2:20): If the Halachah requires Reuven to divorce his wife but he refuses, Beis Din of Yisrael in every time and place hit him until he says that he wants. He writes a Get, and it is Kosher. Ones invalidates an action that the Torah does not obligate one to do, e.g. to sell or give a gift. If one was seized by his Yetzer Ra and enticed to omit a Mitzvah or do an Aveirah, and he was hit until he did his obligation, this is not Ones. He wants to be a Yisrael and do all the Mitzvos. (Until he was hit) he was Ones due to his evil intent!

Source (Magid Mishnah): Rav Huna (Bava Basra 47b) taught that a coerced sale is valid. We conclude that he cannot learn from a coerced Get, for there the husband truly wants to divorce, for it is a Mitzvah to heed Chachamim.

Or Some'ach: Here, Hagahos Maimoniyos says that we force an apostate to Islam to divorce. In Hilchos Ishus, he says that we do not. It seems that we force only if there is reason to divorce in addition to apostasy. However, the Rambam validates the Get because he wants to be a Yisrael. This does not apply here! Even after divorcing he is angry at those who forced him! It is difficult to say that Chachamim uprooted his Kidushin to prevent Igun.

Poskim

Shulchan Aruch (EH 134:4): If one swore to give a Get, they must permit his oath before (he gives it), lest it resemble Ones. If he gave Arvus, he gives if he wants. This is not like Ones.

Beis Shmuel (6): If the Halachah obligates him to divorce, we are not concerned if he additionally obligated himself through an oath.

Rema: The same applies if he agreed through a Kinyan to divorce (Terumas ha'Deshen 173).

Rebuttal (Beis Shmuel 7 and Gra 12): They must pardon the Kinyan, lest the Get be like Ones. This connotes that a Kinyan to give a Get is Chal (takes effect). However, the Terumas ha'Deshen says that if one accepted a Kinyan to give a Get and a fine, since the Kinyan is not Chal, the fine is not Chal! The Rambam (Hilchos Mechirah 5:14) and SMA (195:16) say similarly.

Rema: If he accepted upon himself fines if he will not divorce, this is not Ones, for the Get depends on something else, and he could pay the fine and not divorce.

Gra (13): We learn from Gitin 46b. (If one forbade all produce to himself if he will not divorce, there is no need to decree for Tikun ha'Olam.)

Rema: Some are stringent even in this case. L'Chatchilah, it is good to be concerned and exempt him from the fine. If he already divorced due to it, or even due to his own oath to divorce, the Get is Kosher, for initially he was not forced.

Source (Gra 14): Bava Basra 47b says that a self-induced Ones is not Ones.

Rema: Some say that Ones is only when he himself is coerced, but if they afflicted Yakov until Levi divorced, even if Yakov is Levi's father or son, this is not called Ones. Some are stringent if a father was coerced so the son will divorce.

Shulchan Aruch (7): If Yisre'elim improperly forced one to divorce and he said 'I want', the Get is Pasul, but she is forbidden to Kehunah.

Taz (9): Really, the Get is Batel. We decree to forbid to Kehunah lest it be confused with a properly coerced Get. This concern applies also if another man was Mekadesh her; she should need a Get. However, sometimes we disqualify from Kehunah even when is not a Get at all, e.g. 'you are divorced from me but you are not permitted to any man.' Perhaps also here, we are stringent only about Kehunah.