God tells Jonah to visit Nineveh and preach to them. The Ninevites readily repent, which seems fantastical. The idea that a major city in an empire which controlled most of the Middle East for 250 years would simply just convert on the say so of an Israelite prophet, doesn't make much sense at all.
In fact, Jewish writers in the Middle Ages had pondered whether this was a work of fiction.
Though the Ninevites may had their own reasons.
Who Were the Assyrians?
Nineveh was a city within the Assyrian Empire. The Assyrians had lived in the area of northern Mesopotamia for a millennia. But their greatest achievements occurred in the Neo-Assyrian age between 911 - 609 BC.
The Age of the Magnates
Jonah's ministry coincided with the Age of the Magnates (823-745 BC). After the death of Shalmaneser, a civil war broke out. This coincided with a decline in the empire.
There were no major expansions during this time and the power of the Assyrian King grew week. The Magnates took control of different regions within the Empire. And as Nineveh was not the capital, it was likely the king of Nineveh was one of the Magnates and not the King of Assyria.
Pagans and their Gods
During this period, there were several epidemics and eclipses which fed into the idea that the Assyrian gods had failed the people.
The relationship between the gods and the ancient pagans was not one of complete servitude. It was was more complicated, much more like a quid pro quo.
By building a temple, installing the idol and feeding it, the ancient pagan expected benefits back. When the crops failed, a pandemic struck the land or some other disaster occurred, if the people had followed the correct protocols, it was probable the god who had failed.
Sometimes, new protocols were developed, maybe even more draconian than the last.
Or sometimes the people started searching for a new god or gods.
If the Assyrian Empire was at a low, then it could be conceivable that the inhabitants of Nineveh would accept Yahweh as their new God. A replacement for their Assyrian pantheon.
The King of Nineveh
Jonah travels through the whole of the city. Nineveh was large, and it took a three days journey to walk across it.
As Jonah walked through the city calling for repentance. Eventually the king heard the message and declared a time of sack-cloth and ashes.
At first, this was not a top down repentance, it came from below. Much like how the Roman Empire changed from pagan to Christian. It started from a small ‘inconsequential’ group, who within 400 years took over the Empire.
Nineveh and Nahum
After Jonah's visit, the Assyrian Empire recovered, and Nineveh soon became the capital. But the Assyrians returned to terrorising the surrounding nations.
God was not happy and instructed Nahum to prophecy the demise of Ninevah and the Assyrians.
God showed mercy to Nineveh. He blessed them. They still rejected God, and the Assyrian Empire was finally destroyed in 609 BC.
I feel this is a template to how God deals with the wayward nations. He is compassionate. He is slow to anger. But eventually, He cannot allow the pain and suffering to continue and allows them to be destroyed.
Nahum 1 v 3
The LORD is slow to anger but great in power; the LORD will not leave the guilty unpunished.