Before we mention the books of the Prophet, peace be upon him, to kings and princes, it is worth taking a look at state of the Roman Empire (present-day Italy) Persia (now Iran) The wars were between the Roman Empire and the Persians.
In the year 621 AD
The Persian armies were victorious and seized the Levant, Egypt and Asia Minor, a year before the Hijrah, and the Persians - now Iran - were threatening Constantinople at that time, (Istanbul) Finally, Heraclius appeared (King of the Romans: Italy) and insisted on restoring the glory of his state and in the time of migration (year 622 AD) The Roman Emperor was chasing raiders from Asia Minor, and in the second episode of his incidents, his armies marched into the heart of Persia (Iran) itself, during the three years in which Heraclius was restoring the glory of the empire, the Messenger of God, may God’s prayers and peace be upon him, was in conflict with the Quraysh, and this was followed by the siege of Constantinople (Istanbul) by the Persians, who was half a year before the siege of the city in the battle of the parties (July 626 AD) and in the third battle and to Heraclius his previous victory, so he won a complete victory, on the first of December in the year 626 AD in (the Battle of Nineveh), and thus the armies of the Persians (the Iranians) were defeated and dispersed.
On the twenty-ninth of this month
Chosroes fled to the capital of his kingdom, and in February of the year 628 AD his son (Sheroeh) killed him and seized the throne and concluded a peace treaty with the Roman Emperor, provided that the borders of the two countries remain as they were before, and around this time the Prophet, peace be upon him, was the treaty of Hudaybiyah was concluded with the leaders of Quraysh, and in the spring of this year Heraclius went out to visit Jerusalem
When the Messenger of God returned from the Treaty of Hudaybiyah
He saw the rapid conversion of individuals and tribes, and found that the time had come to spread the call to Islam outside the Arabian Peninsula, so he chose his messengers for that purpose, one of the Muslim merchants who had previously moved to those countries whose kings he wanted to invite to Islam, who knew their customs.
We take advantage of the following
1 - The conflict between civilizations, from the beginning of history until God Almighty inherits the earth and those on it, a year of life that does not change, we find the Persians and the Romans, the kings of the world, fighting over bringing the great civilizations under their banner, such as Egypt and Constantinople (currently Turkey), they reached it or not, the result does not matter, except that the experience is always an honor for kings.
. 2 - The Messenger, until the time of the Hudaybiyah Treaty, a year before the conquest of Mecca and the end of the Muhammadan message, was also in a clash of civilizations - if the expression is right - In the Arabian Peninsula, the Islamic civilization that brought nothing but good for humanity, in addition to the Arabian Peninsula, in which people ate the rights of the weak, and they eat dead meat and other vices, so it is polite, pure, and the best condition of people, until the building of a civilization with its full human standards recognized by it, and it begins to wrestle with Arab civilization in the Arabian Peninsula, And around it, and indeed, many of them dominated and many entered Islam, which is a sign of the emergence of civilization. why? Because as it is said: one of the differences between culture and civilization, that culture is local, if it moves to a place other than where it originated, then it is a civilization.
3- Here, after things have stabilized to a certain extent after the conquest of Mecca, and since Islam is a global religion and not a local (for the Arabian Peninsula or the Arabs) The Messenger of God - may God bless him and grant him peace - began to correspond with kings and princes. This is how political rulers do after the stability of their civilization, to try to assimilate civilizations, which turned them to enter into a general consensus or an agreement not to fight, at the very least.
Related studies
Religion and Power: Divine Kingship in the Ancient World and Beyond
Monarchy, and especially the sacred aspects of the office of monarch, have long fascinated scholars in a variety of fields such as history, religious studies, or area studies, monarchy (or any kind of absolute power) and its close relationship to religion and its use for the purpose of legitimizing power seems to be an almost universal concept in human history, among the oldest civilizations that displayed the phenomenon of god-kings were Mesopotamia and ancient Egypt. So it is all the more surprising that the ancient Egyptian monarchy is ignored - To a lesser extent - and the ancient monarchy in Mesopotamia in comparative studies of the phenomenon of divine or sacred kingship.
الروابط
https://oi.uchicago.edu/research/symposia/religion-and-power-divine-kingship-ancient-world-and-beyond-0 https://ar.wikisource.org/wiki/%D8%B3%D9%8A%D8%AF%D9%86%D8%A7_%D9%85%D8%AD%D9%85%D8%AF#%D8%B1%D8%B3%D9%84_%D8%A7%D9%84%D9%86%D8%A8%D9%8A_%D8%B5%D9%84%D9%89_%D8%A7%D9%84%D9%84%D9%87_%D8%B9%D9%84%D9%8A%D9%87_%D9%88%D8%B3%D9%84%D9%85_%D8%A5%D9%84%D9%89_%D8%A7%D9%84%D9%85%D9%84%D9%88%D9%83_%D9%88%D8%A7%D9%84%D8%A3%D9%85%D8%B1%D8%A7%D8%A1_%D9%8A%D8%AF%D8%B9%D9%88%D9%87%D9%85_%D8%A5%D9%84%D9%89_%D8%A7%D9%84%D8%A5%D8%B3%D9%84%D8%A7%D9%85