In August 629, Muhammad Mustafa, the Messenger of God, addressed letters to the rulers of the neighboring countries inviting them to Islam.
E. Von Grunebaum
In 629, Mohammed sent letters to six rulers - the Persian king, the Byzantine emperor, the Negus of Abyssinia, the governor of Egypt, a Ghassanid prince, and a chief of the Banu Hanifa in south-east Arabia, inviting them to Islam. (Classical Islam - A History 600-1258)
Muhammad was God's Messenger not only for the Arabs but for the whole world. It was his duty to deliver God's last message to all mankind, and he did. Professor Margoliouth, however, considers these letters a prelude to aggression and conquest. He says:
About the time of the campaign of Khaibar, he (Mohammed) published his program of world-conquest by sending letters to the rulers of whose fame he had heard. (Mohammed and the Rise of Islam, 1931)
It is true that the program of Muhammad, the Messenger of God, was one of “world-conquest,” but not by force of arms. His aim was to conquer the minds and the hearts of men and women, which Islam did in his day, and is still doing today.
In sending these letters, the Prophet was prompted by his desire that all men should live in obedience to the commandments and laws of God. Obedience to those commandments and laws alone can guarantee the peace, happiness and welfare of mankind in this world, and its salvation in the Hereafter.