When the Messenger of Allah (sallallaahu 'alayhi wa sallam) sent a letter to Heraclius, the king of Byzantium, telling him to accept Islam he refused.
He refused because he feared that he would lose his empire and would be killed by his people if he accepted. As a result, he ended up a loser in this world and the next. He lost his empire in this world and was from the losers in the next world.
In contrast, when the Messenger of Allah (sallallaahu 'alayhi wa sallam) wrote to Negus, the king of Abyssinia (Ethiopia), he accepted Islam.
And the result was that he was successful in both worlds. He did not lose his empire and when he died the Greatest of creation (sallallaahu 'alayhi wa sallam) offered his funeral prayer when he died.
***Only if Heraclius had really understood the words of the Prophet (peace and blessings of Allaah be upon him) in the letter that he sent to him: “Become Muslim and you will be safe,” and he had understood that in general terms as applying both to this world and the Hereafter, he would have been safe from all that he feared if he had become Muslim. But guidance is in the Hand of Allaah. The Negus, the king of Abyssinia, became Muslim and remained in power. ***