"Fight against those who believe not in Allah, nor in the Last Day, nor forbid that which has been forbidden by Allah and His Messenger and those who acknowledge not the religion of truth among the
people of the Scripture, until they pay the Jizya with willing submission, and feel themselves subdued" Qur'an, Sua 9:293
Thus the reason for fighting the Jews and the Christians according to Zamakhshari is to improve the f,rnancial situation of Muslims.
Baidawi...... they deviated from their own religion, which has been annulled through Islam. And in the next passage he says that Islam "annuls and destroys the other religions". These words suggest that the reason for fighting the People of the Book according to Baidawi is their eroneous practices and beliefs.
Abu al-Ala Mawdudi.......the purpose for which the Muslims are required to fight the People of the Book. And the purpose is "to put an end to the sovereignty and supremacy of the unbelievers so that the latter are unable to rule over men".4
An alternative commentary can be found here: http://www.answering-islam.org/Silas/swordverse.htm