Praise be to Allah.
When Allaah issued the command to fight the Jews and Christians, in the aayah (interpretation of the meaning):
“Fight against those who (1) believe not in Allaah, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allaah and His Messenger (Muhammad), (4) and those who acknowledge not the religion of truth (i.e. Islam) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued” (al-Tawbah 9:29)
-- He then stated the reason that made it obligatory to fight them, which was their disbelief in Allaah and their shirk – which included their speaking ill of Allaah by attributing a son to Him, when He is the One, the Self-Sufficient Master, He begets not, nor was He begotten, and there is none co-equal or comparable unto Him. The Jews said that ‘Uzayr is a son of Allaah, and the Christians said that the Messiah was a son of Allaah. ‘Uzayr was a righteous man from among the Children of Israel, who was venerated by the Jews. It was said that the reason why he was venerated was that he had memorized the Tawraat (Torah), so the Jews, or some of them, exaggerated about him and claimed that he was a son of Allaah. So Allaah condemned them for that and told them that by saying this they were doing the same as the mushrikeen who said that the angels were daughters of Allaah. The Christians also spoke ill of Allaah when they said that the Messiah was a son of Allaah; by doing so they resembled the Jews and mushrikeen. Allaah says (interpretation of the meaning):
“And the Jews say: ‘Uzair (Ezra) is the son of Allaah, and the Christians say: Messiah is the son of Allaah. That is their saying with their mouths, resembling the saying of those who disbelieved aforetime. Allaah’s Curse be on them, how they are deluded away from the truth!” [al-Tawbah 9:30]
See Tafseer Ibn Katheer, the second part of his tafseer of Soorat al-Tawbah; Tafseer Ibn Jareer; Tafseer al-Qurtubi.