What is the meaning of "Embrace Islam and you will be safe" in the Prophet's letter to the Emperor of the Byzantines?

Question

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.

Answer 1

The phrase ⟪Embrace Islam and you will be safe⟫ is a short way of getting across the message.

It includes both safety in the world from war or Jizyah or punishment from Allah and safety in the afterlife from the Hellfire.

The fact that the disbelievers would face punishment for rejecting prophets in this world is not some new concept such that the Prophet needs revelation before he knows it.

Obviously, in the case of Islam, the disbelievers were punished through the swords of the Muslims.

Answer 2

Meaning of this statement can be referred back to Quran and Sunnah.

It is those who believe (in the Oneness of Allâh and worship none but Him Alone) and confuse not their belief with Zulm (wrong i.e. by worshipping others besides Allâh), for them (only) there is security and they are the guided. Quran 6:82

And Allâh puts forward the example of a township (Makkah), that dwelt secure and well-content: its provision coming to it in abundance from every place, but it (its people) denied the Favours of Allâh (with ungratefulness). So Allâh made it taste extreme of hunger (famine) and fear, because of that (evil, i.e. denying Prophet Muhammad(ﷺ) which they (its people) used to do.

Messenger of Allah said:

I have been commanded to fight against people till they testify that there is no god but Allah, that Muhammad is the messenger of Allah, and they establish prayer, and pay Zakat and if they do it, their blood and property are guaranteed protection on my behalf except when justified by law, and their affairs rest with Allah. Source: Ṣaḥīḥ Muslim 21, Grade: Sahih

Security is only established in the lands that implement the legislation of Allah.