In Nahum 3:18, the king of which nation is told his shepherds slumber?

Answer: Assyria

"O king of Assyria, your shepherds slumber; your officers sleep." The book ends saying all who hear of his downfall applaud.

“O king of Assyria, your shepherds slumber; your officers sleep. Your people are scattered on the mountains with no one to gather them. There is no healing for your injury; your wound is severe. All who hear the news of you applaud your downfall, for who has not experienced your constant cruelty?”— Nahum 3:18-19 (BSB)