In the sixth year after Hijrat, the Holy Prophet (pbuh) decided to invite neighboring rulers and kings to embrace Islam.
Preparations for sending letters with special messengers began in 6 A.H.
The actual dispatch of messengers took place in 7 A.H.
The Holy Prophet (pbuh) sent letters to several rulers, including Heraclius, the Roman Emperor.
Heraclius had the Holy Prophet's (pbuh) letter read out to him and later sent for Abu Sufyan.
Abu Sufyan was one of the Holy Prophet's (pbuh) bitterest enemies at the time.
Heraclius asked Abu Sufyan questions about the Holy Prophet (pbuh) and was impressed by his answers.
Heraclius remarked that the person who claimed to be a Prophet did possess the attributes of one.
He sent a polite reply but did not embrace Islam.
The Holy Prophet (pbuh) also sent a letter to Chosroes, Emperor of Persia.
Chosroes insulted the Holy Prophet's (pbuh) messenger and tore his letter to pieces.
When the Prophet (pbuh) was informed of this, he said that Chosroes' kingdom would soon be broken up.
This prophecy was fulfilled when Chosroes was killed by his son, who then ascended the throne but later died by poisoning himself.
The Holy Prophet (pbuh) sent a letter to Negus, Emperor of Abyssinia, who embraced Islam and sent presents for the Holy Prophet (pbuh).
The Holy Prophet (pbuh) also sent a letter to Cyrus, Ruler of Egypt, who treated the messenger with great honor and sent a polite reply and presents.
The presents included Hazrat Maria, whom the Holy Prophet (pbuh) later married, and a horse named Duldul.
Cyrus did not accept Islam.
The Holy Prophet (pbuh) sent letters to the Rulers of Bahrain and Oman, both of whom embraced Islam.