God was patient for 100 years then Nineveh was destroyed - why?
The prophecy of Nahum needs to be taken with that of Jonah. After Jonah’s ministry in Nineveh, the city was covenanted to the Lord. After a couple of generations, however, the Assyrians reverted to their old ways. When people have a knowledge of God and then rebel against Him, they become worse than they were before. The Assyrians became known throughout the ancient world for great cruelty in warfare.
The Assyrian empire dominated the ancient Near East for a while, and both Israel and Judah were forced to recognize its superpower status and pay tribute. When Hoshea, the last king of Israel, conspired with Egypt and withheld the annual tribute, the Assyrians conquered and destroyed the nation. The larger reason Israel was destroyed was that they had provoked the Lord beyond the limits of His forbearance.
About a century later, God raised up Nahum to predict Nineveh’s doom. Nahum reminded the Ninevites that “the LORD is good, a refuge in times of trouble. He cares for those who trust in Him” (Nahum 1:7). God had loved and protected the Ninevites of an earlier generation (Jonah 4:11), but now the nation had rejected Him, and had continued in obstinacy for over a century. The Ninevites had seen God’s patience with Israel run out, but they failed to learn from it. Then God’s patience with Assyria had run out, and they too would experience His wrath.
God was angry at Nineveh for rejecting Him, and because they had become an exceptionally cruel people. From what Nahum writes, however, it seems that God was most angry because the Assyrians had attacked Judah, the seat of His government and witness (Nahum 1:12–15). God became furious when His church, His covenant ones were attacked.
Although all three chapters of Nahum are about Nineveh, and his prophecy was doubtless sent to the Assyrians, the primary audience seems to be Judah. The southern kingdom, Judah, also turned its back upon God. Nahum wrote his prophecy in the early days of Josiah’s reign. The destruction of Nineveh was to be a sign to Judah that God’s patience does run out. With his colleagues Zephaniah and Jeremiah, Nahum was calling Judah to repentance.
(1991). Tabletalk Magazine, December 1991: Worship: Heralding His Glory, 31.