NOTES for Jon 3:4-5
It appears that Jonah's preaching was successful, if he was able to bring even the inhabitants of a city like Nineveh to repentance. Indeed, in the pre-exilic period (when the Book of Jonah was written), Nineveh was the symbol of everything worst in the world (after the Babylonian exile, Nineveh's place in the biblical books was taken by Babylon). This is not surprising, since it was the capital of the Assyrian Empire, whose rulers and soldiers stood out for their cruelty even in the ancient world, where cruelty in general was hard to make remarkable. But the matter was not only the legendary cruelty of the Assyrians; it was also the spirit of the empire they created.
In essence, Assyria gave the world the first example of a practically nonreligious society. Of course, the Assyrians, like all peoples of antiquity, had their own gods, whose honor and dignity they zealously protected. But these gods were for the Assyrians symbols of imperial power rather than objects of religious worship. The situation resembled the state of affairs in the late Roman Empire: of course, no one seriously believed in Jupiter Capitolinus anymore, but failing to render him the proper honors meant humiliating Rome, with all the consequences that followed.
A very remarkable witness to this is the speech of the commander of the Assyrian expeditionary corps sent to conquer Egypt, who on the way decided to put an end to Judah's relative autonomy and besieged Jerusalem (it can be found in the Book of Isaiah). He speaks not about the greatness of the Assyrian gods, but about the power of the Assyrian Empire.
He says to the besieged: what God are you hoping in? How many gods have different peoples had, and did even one god ever help anyone? For war, according to him, what is needed is "counsel and strength," that is, a strong army and effective administration, not the support of some gods. These speeches clearly reveal the practical atheism of the speaker, whether or not it was officially declared. And it is reasonable to think that such a worldview was characteristic not of the commander of the expeditionary corps alone.
To make people of this kind repent, one had to make an effort. More than that, a special, purposeful effort by God was needed to achieve such an effect. But the author of the Book of Jonah, as can be seen, understood well: the only possibility for Nineveh and for all Assyria to survive was conversion and repentance for the sins committed. And the historical fate of Assyria, which as a whole proved incapable of such repentance, confirms that he was right.