Nahum prophesied that the Assyrian capital, Nineveh, would be destroyed by who or what?

Nineveh was destroyed by the Medes and the Chaldeans.

Explanation:

The capital of Assyria that was along the Tigris River. It was the largest city of its day spanning three miles long and a mile wide all contained within great walls.

Nineveh was destroyed shortly after Ashurbanipal's death in 612 BC.