Why did Boniface VIII oppose kings taxing the clergy?

Pope Boniface VIII opposed the taxation of the clergy by secular rulers primarily because he viewed the practice as a violation of canon law and a threat to the independence of the church.1

During his pontificate, Boniface sought to prevent the wealth of the church from being used to finance the military conflicts between European monarchs, such as the war between France and England. He refused to allow the papacy to be sidelined while the church’s resources were drained to the prejudice of its own interests.1

To assert papal authority, Boniface issued the papal bull Clericis laicos in 1296, which strictly forbade the taxation of the clergy by lay authorities without express papal consent, under the penalty of excommunication. This stance was part of a broader commitment to maintaining papal, rather than secular, control over the clergy, a principle he later reinforced in the bull Unam sanctam by asserting the supremacy of spiritual power over temporal power.213