“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.