“Fadak” was a flourishing land in which was taken over by the Muslim army without any military force in the seventh year of hijrah after the fortresses of Khaybar were conquered one after another. As a result of these lands falling into the hands of the Muslim army without any fighting, according to the Quran, they belonged to the prophet (pbuh) himself, who granted them to his daughter, Lady Fatimah (as) as the verse “وَ آتِ ذَا الْقُرْبى حَقَّه” had asked him to do so.
Nevertheless, Lady Fatimah (as) was deprived of her personal property after the death of her father (pbuh), and it wasn’t even returned to her children during the reign of Imam Ali (as). Imam Kadhim (as) was asked about why Imam Ali (as) didn’t take Fadak back. The imam (as) answered: “We are the guardians of the people; if anyone is ever deprived of his/her rights, we try to get them back, but we don’t try to get our own rights back.”
To put it in one sentence, the main reason why Imam Ali (as) didn’t repossess Fadak can be that some would consider their khalifah striving for his personal benefits in a time where the situation of the Muslim nation was a serious and sensitive one full of battles and sabotage. The situation then was so complex to the extent that the imam (as) couldn’t solve many of the main problems of the nation, so it wouldn’t make much sense for him to go after getting back Fadak which wasn’t one of them.
“Fadak” was a thriving and flourishing village located near Khaybar, 140 kilometers from Medinah. In the seventh of hijrah, the fortresses of Khaybar were conquered one after another and the central power of the Jews was overthrown. The residents of Fadak surrendered and promised to give the prophet (pbuh) half of their lands and orchards on condition of no fighting and keep the rest for themselves. In addition, they accepted to do the farming of his lands for a wage.
Lands that fall into the hands of the Muslim army without any fighting and violence become the personal property of the prophet (pbuh), and he can make any decisions about them, that is why when the verse: “وَ آتِ ذَا الْقُرْبى حَقَّهُ وَ الْمِسْکینَ وَ ابْنَ السَّبیلِ وَ لا تُبَذِّرْ تَبْذیرا” was revealed unto him, he summoned his daughter and granted her Fadak.
But unfortunately, after the prophet (pbuh) passed away and during the reign of Abu Bakr, Lady Fatimah (as) was deprived of the gift of Fadak.
In his famous book of Sahih Muslim, Muslim ibn Hajjaj Neyshabouri narrates the story of Lady Fatimah claiming Fadak in detail and has reported Ayishah saying that after the khalifah refused to return it to her, she sulked and didn’t speak another word with him until her demise.
It has also been stated in the Nahjul-Balaghah that: “Of course, all that we had in our possession under the sky was Fadak, but a group of people felt greedy for it and the other party [its rightful owners; Imam Ali (as) and Lady Fatimah (as)] withheld themselves from it. Allah is, after all, the best arbiter.”
In order to get the answer to your question, one fact that should be paid attention to is that the imam’s first and foremost priority always, was to preserve Islam itself, despite all of his objections to those in power before him, and that is why he would cooperate with them and help them in internal affairs and governing the Muslim nation, as he himself beautifully put it: “We [the progeny of the prophet (pbuh)] have a right [which was to be the true successors to the prophet (pbuh)] in which if we are allowed to exercise, then all the better, and if not, we prefer to be the second person sitting in the back of the camel [behind the person guiding it, instead of completely getting off]”.
Therefore, in reality, the imam considered political power a tool and means of fulfilling godly objectives, not a goal, and that is why he would prefer to keep quiet about many different issues, both during his reign and the reign of those before him, leaving judgment for future generations to come. As for why he didn’t act according to his own viewpoint during his own rule, although he had the authority to do so, a small example will clarify things. During his own rule, when he attempted to bring an end to the “tarawih” prayer [that had been innovated by one of previous khalifahs and wasn’t a tradition of the prophet (pbuh)], he was confronted with objections and forced to leave the people to themselves. Also, you surely know of his discontent regarding the arbitration of Abu Musa Ash’ari and that he was forced to give in to it. Essentially, the imam’s coming to power was preceded by twenty five years of continuous justification of all the actions and things the previous khalifahs had done, making it almost impossible to oppose their methods and bring change to some of them; one of those being returning the Fadak to its rightful owners, because some would think that the imam was making use of his power for his own benefit. Add to that the fact that Fadak was important to the household of the prophet (pbuh) and Lady Fatimah (as) because it was a gift and remembrance from him and more importantly, a financial asset and backing for them, especially Ali (as), and that is why the government of the time confiscated it; because it was their financial support; doing so would ensure that Ali (as) wouldn’t be able to do anything against them. Keeping in mind all of these and other circumstances, such as the battles and sabotage the nation was experiencing, preventing the imam (as) from making even important and primary changes that the nation was in need of, how was Ali (as) to take back Fadak? It would surely harm the Muslim nation and shadow over more important national issues, and that is why he chose not to.
Hadiths from the imams somewhat point to these issues:
1- Time had passed since the incident [of Fadak’s usurpation] and there was no need for Ali (as) to speak of it after so many years:
Abu Basir says: “I asked Imam Sadiq (as) why Imam Ali (as) didn’t repossess Fadak after he came to power. The imam (as) answered: “Because both the oppressed [Lady Fatimah (as)] and oppressor [those who deprived her of Fadak] had both died and Allah (swt) had punished the oppressor and rewarded the oppressed by then, and Ali ibn Abitaleb didn’t like the idea of returning a property in which its usurper had been punished and the one usurped from had been rewarded already.”
2- Sacrificing personal benefits for higher and universal goals
Ibn Ibrahim Karakhi says: “I asked Imam Sadiq (as) about why Imam Ali (as) didn’t return Fadak after becoming khalifah. He answered: “When the prophet (pbuh) conquered Mekkah, he was asked if he would return to his homeland. He said: Aqil has sold my house. The people asked: Why don’t you take it back? He answered: We belong to a household that doesn’t take back what has been wrongfully taken from them; Imam Ali (as) did the same in order to have followed the prophet (pbuh) [in not taking back what rightfully belonged to him].
A person asked Imam Kadhim (as) the same question; the imam answered: “Our household [the progeny of the prophet (pbuh)] whose guardian is Allah (swt); He is the one who makes sure what belongs to us comes back to us, and we are the guardians of the people and make sure what belongs to them returns to them, but we don’t take back what belongs to us.”
Having said that, it’s good to see what the fate of Fadak was and what happened to it after the martyrdom of Imam Ali (as).
What historic records says is that after Mu’awiyyah took power, he divided it amongst Marwan, Amr ibn Uthman and his son Yazid. During Marwan’s rule, he took control of all of Fadak and he granted it to his son Abdul-Aziz, who later granted it to his son, Umar. Umar ibn Abdil-Aziz gave Fadak back to the descendants of Lady Fatimah (as). After his death, it once again fell into the hands of the Umayyid dynasty and remained there.
After rule of the Muslim empire shifted to the Abbasid dynasty, it was once again given back to Abdullah ibn Hasan [from the progeny of Imam Hasan (as)]. Mansour Davaneqi took it back from them when he took over, but his son returned it to them after his death.
After Mahdi, the son of Mansour, Musa and Haroun took it back again, but after them Ma’moun officially returned it to the progeny of Fatimah (as). It continued to be returned and taken after Ma’moun.
During that era, Fadak was considered a political issue, and wasn’t looked at as a financial asset, because the khalifahs of both dynasties were in no way in any need of the profits it yielded; that is why when Umar ibn Abdil-Aziz returned it, the Umayyids scolded him, saying: “By doing so, you have rejected Abu Bakr and Umar [who had refused to return it during their time]!”
Eventually, during the Abbasid ruler, Mutawakkil, it was seized, its trees cut down by the order of a person by the name of “Abdullah ibn Umar Mazyar”. This shameful act remains a sign of disgrace for the wrongdoers of that time. It must be noted that even the eleven date palms that the prophet (pbuh) had planted with his own hands were cut down. History says Bashran ibn abi Umayyah Thaqafi, the person who had cut them down, suffered from paralysis after returning to Basrah.
For further information, see:
1- The silence and not opposing of Imam Ali (as) during his rule, Question 1585 (site: 2851).
2- Fadak and women not inheriting land, Question 3020 (site: 3666).
 See: Tabarsi, Majma’ul-Bayan, vol. 3, pg. 411.
 Sharhe Nahjul-Balagheh, vol. 16, pg. 274.
 Sahih Muslim, vol. 3, pg. 1380.
 Nahjul-Balaghah, letter 45.
For more information on this topic, you can refer to question 1351 (site: 1450) of this website.
 Nahjul-Balaghah, pg. 472.
 Ibn Abil-Hadid, Sharhu Nahjil-Balaghah, Library of Ayatullah Mar’ashi Najafi, vol. 12, pg. 283.
 With help from Question 1585 (site: 2851).
 “فَقَالَ لَهُ لِأَنَّ الظَّالِمَ وَ الْمَظْلُومَةَ قَدْ کَانَا قَدِمَا عَلَى اللَّهِ عَزَّ وَ جَلَّ وَ أَثَابَ اللَّهُ الْمَظْلُومَةَ وَ عَاقَبَ الظَّالِمَ، فَکَرِهَ أَنْ یَسْتَرْجِعَ شَیْئاً قَدْ عَاقَبَ اللَّهُ عَلَیْهِ غَاصِبَهُ وَ أَثَابَ عَلَیْهِ الْمَغْصُوبَةَ” Biharul-Anwar, vol. 29, pg. 395, hadith 1.
 Ibid, hadith 2.
 Ibid, hadith 3.
 Jafar Sobhani, Furughe Abadiyyat, vol. 2, pg. 669.
 Ahmadi Miyanji, Makatibul-Rasul.