Nineveh was the capital of Assyria, which was a great empire for 300 years. The events in the book of Jonah seem to have taken place during a period of Assyrian stagnation (783-745) when the Assyrian kings were mostly ineffective. This was the same time period as Amaziah and Uzziah in Judah, and Jeroboam II in Israel (2 Kings 14:23-29). Jonah was able to capitalize on the declining conditions in Nineveh, and saved an entire city of clueless people (4:11).
The wearing of sackcloth was occasionally practiced among biblical people. From as early as the time of Job (Job 16:15) and Jacob (Genesis 37:34), people would put on sackcloth in times of mourning. Perhaps the discomfort of having burlap next to one’s skin amplified the feeling of loss that the wearer was experiencing. Whatever the reason, the Bible tells us that people did this when dealing with a profound loss.
It is interesting to note that Jonah’s work in Nineveh led to the eventual destruction of the northern kingdom of Israel. If Nineveh had not repented, then they would have been destroyed, and the Assyrians would not have eventually conquered Israel and taken them into captivity.
Jonah’s message to Nineveh was that they were at risk of being “overturned” (NIV). The Hebrew word has multiple meanings. It could refer to destruction, or being overpowered by another, or it could refer to a transformation of some kind. Looking at it that way, Jonah’s prediction of 3:4 came true. There was a transformation that led God to refrain from destroying that great city (3:10).