The Prophet had sent the letter to Heraclius:
In the name of Allah, the most Beneficent, the most Merciful. From Muhammad, the slave of Allah, and His Apostle, to Heraculius, the Ruler of the Byzantine. Peace be upon the followers of guidance. Now then, I invite you to Islam.
Embrace Islam and you will be safe, embrace Islam and Allah will bestow on you a double reward. But if you reject this invitation of Islam, you shall be responsible for misguiding the tillers.
‘O people of the Scriptures! Come to a word common to you and us and you, that we worship. None but Allah, and that we associate nothing in worship with Him; and that none of us shall take others as Lords besides Allah. Then if they turn away, say: Bear witness that we are (they who have surrendered’(3.64)
[ Sunnah Ref: Sahih al Bukhari 2940 ]
and my question is about the meaning of "Embrace Islam and you will be safe". I am not sure if the prophet is threatening him with invasion or he means that he will be safe from the punishment of the afterlife, for having died upon rejection of Islam. Or both. Currently I think the 2nd is more likely, because the Prophet did not mention the dhimmi option that would be available to the christian byzantines in the case of invasion. But then again, 9/29 was not revealed yet because this letter was sent during the period the treaty of hudaybiyah was active. And this would also mean Offensive Jihad was not yet legislated (I could be wrong about this, perhaps another command for offensive jihad was already declared) Even if that is correct, the Prophet could may have already known that the scope of warfare would expand to include all kuffar, so he pre emptively warned Heraclius about it.
This is all just mere conjectures on my part, maybe a more solid answer is available from you guys.