The relationship between divorce and Ila':
The prohibition that results from marriage by the power of marriage is of four types: Divorce, Ila', Li`an (disprove a charge of adultery made by the husband against his wife), and Zhihar (a man likening his wife to an unmarriageable relative).
So, prohibition starts with divorce because it is the origin and permissible for a husband to do.
Then the less degree of permissibility is Ila' because it is a permissible oath but in the meantime it includes the meaning of oppression so it is prohibited. The reason of Ila' is the same of revocable divorce which is incompatibility, and they are alike because the cut of the relation is timed by a certain time but some people choose revocable divorce because taking the wife back in marriage will not imply a dislikable matter.
Some people choose Ila' because it does not entail the decrease of the times of divorce unlike the case of revocable divorce.
Qatadah said: Ila' was a type of divorce for the people of the Pre-Islamic Period. Sa`id ibn Al Musayyab said: Ila' was one of the tricks which the People of the Pre-Islamic Period used to practice when a man disliked his wife and did not want to marry another, so he would swear not to approach her, leaving her neither divorced nor married. The people of the Pre-Islamic Period used to practice it but when Islam came, Allah fixed a certain period for it. The definition of Ila' in linguistics is: It is an expression of oath.
In the Shari`ah: It is an oath made by a husband in order not to approach his wife for more than four months or connecting between approaching her and a certain action.
(1) See: Al Bahr Ar-Ra'iq 4/65, Al `Inayah Sharh Al Hidayah 5/441, Ad-Dur Al Mukhtar 3/422, Al Istidhkar 6/35, Al Mudawwanah 2/366, Al Kafi in the Fiqh of the People of Medina 2/598, I`anat At-Talibin 4/33, Al Um 5/265, Rawdat At-Talibin 8/229, Al Mughny 8/505 - 508, Hashiyat Ar-Rawd Al Murbi` 6/619, Al Fatawa Al Kubra 3/275, 276 and 5/504, the books which contain the letter and Fatwas of Ibn Taymiyah 20/381, Zad Al Ma`ad 5/344, Tafsir At-Tabary 4/456, Tafsir Al Baghawy 2/36, Tafsir Ath-Tha`laby 1/266, Tafsir Ibn Kathir 1/604, Tafsir Al Alusy 2/232.