This mighty empire dominated the Middle East from the 9th to the 7th century BC, extending from the Persian Gulf, across the fertile crescent and south to Egypt, having conquered what is now Iraq, Syria, Jordan and Lebanon. The Assyrians were a fierce and cruel nation who showed little mercy to those they conquered (2 Kgs 19:17). A (generally) rival empire was Babylonia.
God directed Jonah to travel to Nineveh, the capital city, and preach about His impending judgment. Jonah didn’t want to obey, yet around 759 BC when he eventually did, the king of Assyria and the entire city of Nineveh repented and God delayed His judgment for a time (Jnh 1:1-3, 3:10). In this intervening period, the Northern Kingdom of Israel was captured by Assyria and its inhabitants taken into exile (2 Kgs 15:29; 1 Chr 5:26). Some 12 years later (in 721 BC) Samaria (Israel’s capital at that time), was besieged and later fell, fulfilling Isaiah’s prophesy that God would use Assyria as “the rod of His anger” for their idolatrous lifestyle (2 Kgs 17:15-17; Isa 10:5-19). This shows us that God can use anyone or anything to further His purposes.
Around 701 BC, when the Assyrian army occupied Judah right to the outskirts of Jerusalem, Hezekiah prayed and God promised him the enemy would not set foot inside the city. God slew 185,000 of the Assyrian forces in one night and several years after slinking home the King was killed by his own sons (2 Chr 32:20-22; Isa 36:4-21, 37:33,36,38). Thus the promised judgement started and in 612 BC Nineveh was besieged by other nations and completely destroyed and Assyria crumbled.