This tax was not imposed on the Christians, as some would have us think, as a penalty for their refusal to accept the Muslim faith, but was paid by them in common with the other dhimmīs or non-Muslim subjects of the state whose religion precluded them from serving in the army, in return for the protection secured for them by the arms of the Musalmans. When the people of Hīrah contributed the sum agreed upon, they expressly mentioned that they paid this jizyah on condition that “the Muslims and their leader protect us from those who would oppress us, whether they be Muslims or others.” [98] Again, in the treaty made by Khālid with some towns in the neighbourhood of Hīrah, he writes: “If we protect you, then jizyah is due to us; but if we do not, then it is not due.” [99] How clearly this condition was recognised by the Muhammadans may be judged from the following incident in the reign of the Caliph ʻUmar. The Emperor Heraclius had raised an enormous army with which to drive back the invading forces of the Muslims, who had in consequence to concentrate all their energies on the impending encounter. The Arab general, Abū ʻUbaydah, accordingly wrote to the governors of the conquered cities of Syria, ordering them to pay back all the jizyah that had been collected from the cities, and wrote to the people, saying, “We give you back the money that we took from you, as we have received news that a strong force is advancing against us. The agreement between us was that we should protect you, and as this is not now in our power, we return you all that we took. But if we are victorious we shall consider ourselves bound to you by the old terms of our agreement.” In accordance with this order, enormous sums were paid back out of the state treasury, and the Christians called down blessings on the heads of the Muslims, saying, “May God give you rule over us again and make you victorious over the Romans; had it been they, they would not have given us back anything, but would have taken all that remained with us.” [100] As stated above, the jizyah was levied on the able-bodied males, in lieu of the military service they would have been called upon to perform had they been Musalmans; and it is very noticeable that when any Christian people served in the Muslim army, they were exempted from the payment of this tax. Such was the case with the tribe of al-Jurājimah, a Christian tribe in the neighbourhood of Antioch, who made peace with the Muslims, promising to be their allies and fight on their side in battle, on condition that they should not be called upon to pay jizyah and should receive their proper share of the booty. [101] When the Arab conquests were pushed to the north of Persia in A.H. 22, a similar agreement was made with a frontier tribe, which was exempted from the payment of jizyah in consideration of military service. [102]