Why did Ninevites respond the way they did?

I'm reading through the book of Jonah, and in an historical context it doesn't make much sense to me. Nineveh was part of Assyrian empire. It practised Ashurism focused on worship of of Ashur and Inanna. When God sent Jonah to Nineveh I expected the response to be something like: > "Who is the LORD, that I should obey him ..." Exodus 5:2 Assyrians are also said to have practised homosexuality, which was contrary to Judaism. To me it seems unlikely that an entire city would abandon their religion and established customs instantly on the words of an unknown man calling himself a prophet of some foreign religion. Is the book of Jonah describing a historical event?