"...Really, the one who follows the Noble Qur'an finds that the pages dedicated to the People of the Book are greater than their size and influence in the Arabian Peninsula. So, what is the secret behind this disproportional focus on them?
Allah Knew that after the idolaters were pushed out of the way of the spread of this call, the People of the Book would in the future take their place as the lump in the throat standing in the face of this religion. And Allah Knew that the Jews would head all efforts in fighting and opposing this religion since their days in Madinah all the way to the Day of Resurrection. Because of this, we have these long pages focusing on the People of the Book.
In truth, after polytheism was eradicated from the Arabian Peninsula, issues began with the People of the Book, and wars ensued with the People of the Book, and battles continued with the People of the Book, and the majority of battles that the Muslims fought were against the Romans. The Persians were finished after the defeat of Kisra, after the battle of Nahawand that was commanded by an-Nu'man bin Muqrin. The Persian state was finished, and this was prophecized by the Messenger of Allah (P.B.U.H) when he sent a letter to Kisra and a letter to Heraclius, and Kisra tore up his letter, and the Prophet said: "May Allah tear up his kingdom," and he said: "If Kisra is destroyed, there will be no Kisra after him." The Persian empire was unable to stand up in front of the Muslims for more than ten years or so.
As for the Romans who have been fighting the Muslims since the days of Yarmuk until today, they escaped to Constantinople and took it as a base from which to fight this religion, and they stood as a preventative barrier to the spread of this religion. In Rome, the Western Church was the Pope's base, and Charles Martel stood in the face of this religion in the Battle of Tours in Europe after he had killed 'Abd ar-Rahman al-Ghafiqi in the Martyrs' Court in France. So, the advance in Europe was halted until the Turks came once again and conquered Constantinople, and the battles started all over again. Muhammad al-Fatih came and conquered Constantinople and began to initiate another conquest within Europe. With this, he faced immense challenges from the People of the Book, while the idolaters - the Persians and Magians - are for the most part finished..."
['Fi Dhilal Surat at-Tawbah'; p. 140-141]
(SHEIKH 'ABDULLAH YUSUF 'AZZAM (ASY-SYAHID, INSYAALLAH)