Is fighting the People of the Book superior to fighting others?

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Fighting the People of the Book is superior to fighting other disbelievers. This superiority is supported by the report that the Prophet peace be upon him told Umm Khallad that her son received the reward of two martyrs because he was killed by the People of the Book (Hadith narrated by Abu Dawud). Ibn al-Mubarak preferred campaigning against the Romans (who are People of the Book) over fighting disbelievers near him in Marw because they fought based on a religious doctrine.