In theory all ecclesiastical persons and possessions were immune from secular taxation, but the pope frequently permitted temporal rulers to levy a tax on them for the aid of the state in times of public necessity. At the command of the pope such taxes had been assessed (1) to carry on the crusades (the Saladin tithe), (2) to make war on Frederick II, (3) to put down the heresy of the Albigenses, (4) to resist Peter of Aragon when he attacked Sicily, etc. It frequently happened that the large sums raised for the crusades went into the king's treasury, and were spent for other things. The kings, especially of England and France, found this a very convenient way of raising money. The immediate cause of the publication of this bull was the heavy assessments which the kings of England and France had just made on their clergy. Boniface recognized that the immunities and liberties of the church were thereby being destroyed. In spite of the protests of both pope and clergy, neither king restored the money or ceased to levy taxes. New names for them were so skilfully invented, and such arguments were used, that the clergy could not refuse to pay without seeming to be disloyal and unpatriotic. Boniface VIII issued this bull to put a stop to the taxation which he regarded as the pillaging of the churches. It must be observed that the pope does not prohibit such taxes altogether. He preserves his authority and the immunities of the church by retaining the right to sanction whatever taxes may be assessed on the clergy and the possessions of the church. The kings of both England and France were engaged in policies which necessitated large expenditures, and hence they were in need of money. Besides, they were trying to centralize all authority in their hands and consequently found these ecclesiastical immunities a great obstacle in their way. We have here an evidence that the national governments had begun their long struggle against the temporal authority of the pope, for the question as to whether the king may tax the church and clergy was one phase of this struggle.