Inscriptions by King Ashur-nâsir-pal II (884 859 BC)
I borrowed a book from my university library that has a whole chapter of 61 pages full of translations of the annals of Ashurnasirpal II. They deal mostly with his wars and conquests. The excerpts below give a good idea of the extreme cruelty and violence of the Assyrians, and the whole chapter is full of similar accounts. Time and again the annals of Ashurnarsipal (and pretty much all the Assyrian kings) relate how he went to this or that city and laid it waste, burning it with fire and killing and deporting its inhabitants. In a very few cases where the city surrenders without a fight and kisses his feet, he "has mercy" on them and just destroys and plunders the city without killing too many people, but usually he is very bloodthirsty.
In the light of their fearsome reputation, it is perhaps not too surprising that Jonah was not too eager for God to spare the Assyrians. The fact that God had mercy towards such a people also shows the very long-suffering nature of God.
On the other hand it also shows that it is terrible to face the wrath of God, for it was the terrible Assyrian war machine that God chose to use as, "the rod of my anger, in whose hand is the club of my wrath!" (Isaiah 10:5). God used the Assyrians to punish the northern kingdom of Israel, just as He had earlier used the children of Israel to punish the Canaanites (see Deut 9:5-6). But in turn, after using the Assyrians to punish the kingdom of Israel, God would punish the Assyrians for their cruelty and pride (Isaiah 10:5-19). In the wars and conflicts that we see around us in the world today, perhaps similar principles are being acted out.
Although the king of Assyria and the people of Nineveh repented when Jonah reluctantly preached to them, and God spared the city at that time, it was not long before they went back to their old ways, and so eventually Nineveh was destroyed in 612 BC, by a coalition of Medes and Babylonians. These peoples had earlier been subjected to the merciless onslaught of Assyrian armies and they did not forget that. They did to Nineveh what the Assyrians had done to them, and totally destroyed it with great bloodshed. This was foretold by the Prophet Nahum in words reminiscent of the Assyrians' descriptions of their own exploits: "Woe to the city of blood, full of lies, full of plunder, never without victims! The crack of whips, the clatter of wheels, galloping horses and jolting chariots! Charging cavalry, flashing swords and glittering spears! Many casualties, piles of dead, bodies without number, people stumbling over the corpses.... ....O king of Assyria, your shepherds slumber (note: "wonderful shepherd" was one of the titles of the the Assyrian kings, frequently used in their inscriptions); your nobles lie down to rest. Your people are scattered on the mountains with no one to gather them. Nothing can heal your wound; your injury is fatal. Everyone who hears the news about you claps his hands at your fall, for who has not felt your endless cruelty?" (Nahum 3:1-3, 18-19).
And now for the excerpts from the book: Luckenbill, Daniel D., Ancient Records of Assyria and Babylonia, Vol. 1: Historical Records of Assyria, from the earliest times to Sargon, (Chicago: University of Chicago Press) 1926.
pp. 146-147 Annals of Ashur-nârsi-pal II
445. ....At the source of the river Subnat, where stand the images of Tiglath-pileser and Tukulti-Urta, kings of Assyria, my fathers, I fashioned an image of my royal person, and I set it up beside them. At that time I received the tribute of the land of Isala,cattle, flocks, and wine. To the mountain of Kashiari I crossed, to Kinabu, the fortified city of Hulai, I drew near. With the masses of my troops and by my furious battle onset I stormed, I captured the city; 600 of their warriors I put to the sword; 3,000 captives I burned with fire; I did not leave a single one among them alive to serve as a hostage. Hulai, their governor, I captured alive. Their corpses I formed into pillars; their young men and maidens I burned in the fire. Hulai, their governor, I flayed, his skin I spread upon the wall of the city of Damdamusa; the city I destroyed, I devastated, I burned with fire. The city of Mariru, which was within their borders, I captured. Fifty of their warriors I put to the sword; 200 of their captives I burned with fire. 332 men of the land of Nirbu I slew in a battle on the plain; their spoil, their cattle, and their sheep I carried off. The (men of the) land of Nirbu, which is at the foot of Mount Uhira, had banded themselves together, and had entered the city of Tela, their stronghold. From Kinabu I departed, to the city of Tela I drew near. The city was exceeding strong and was surrounded by three walls. The men trusted in their mighty walls and in their hosts, and did not come down, and did not embrace my feet. With battle and slaughter I stormed the city and captured it. 3,000 of their warriors I put to the sword; their spoil and their possessions, their cattle and their sheep I carried off. Many captives from among them I burned with fire, and many I took as living captives. From some I cut off their hands and their fingers, and from others I cut off their noses, their ears, and their fingers(?), of many I put out the eyes. I made one pillar of the living, and another of heads, and I bound their heads to posts (tree trunks) round about the city. Their young men (Col. II) and maidens I burned in the fire, the city I destroyed, I devastated, I burned it with fire and consumed it. At that time the cities of the land of Nirbi and their strong walls I destroyed, I devastated, I burned with fire.
From the Kurkh monolith inscription:
....All night I marched, I drew nigh unto Pitura, the stronghold of the men of the city of Dirra. The city was exceeding strong, was surrounded by two walls, its citadel was built like a mountain peak. (Trusting in) the supreme might of Assur, my lord, with the masses of my hosts and with my furious onslaught I battled [with them]. For two days, from before sunrise, I thundered against them like Adad, (the god) of the storm, and I rained down flame upon them. [With courage] and might my warriors flew against them like Zû. I captured the city, 800 of their warriors I struck down with the sword, [I cut off] their heads. Many [of the men] I captured alive, the rest of them I burned with fire, their heavy spoil I carried off. A pillar of living (men and) of heads I built in front of their city gate,  men I impaled on stakes in front of their city gate. The city I destroyed, I devastated, I turned it into mounds and ruins; their young men I burned in the flames. The city of Kukunu, which is situated at the entrance (lit., mouth) of the pass of the mountain of Matnu, I captured, 700 of their warriors I struck down with the sword, their great spoil I carried off.
500. Forty cities of the land of Dirra I captured, and I slew their inhabitants, I carried off their spoil; forty men I captured alive, the cities I destroyed, I devastated, I burned with fire. The (terrifying) splendor of my dominion I poured over them. From the city of Pidara I departed, to the city of Arbaki, of the land of Kirhi, which is on the inside, I went down. At my royal splendor they were affrighted, and their strong, walled cities they cast down; to save their lives, into the mighty mountain of 'Matna' they went up. I marched after them, 1,000 of their fighting men I slaughtered in the midst of the steep mountain, with their blood I dyed the mountain, with their corpses I filled the gullies and precipices of the mountain. 200 men I captured alive, I cut off their arms, and 2,000 of them as captives I carried away; their cattle and sheep in countless numbers I carried off. Iiaia and Salanibe, the strong cities of the land of Arbakki, I captured, I slew their inhabitants and I carried off their spoil.
501. 250 strong, walled cities of the lands of Nairi I destroyed, I laid waste, into mounds and ruins I turned them. The harvest of their land I gathered, grain and straw I heaped up in the city of Tushha.....
Links to photos of Ashurnasirpal artifacts: