"The burden against Nineveh... God is jealous, and the Lord avenges; the Lord avenges and is furious. The Lord will take vengeance on His adversaries, and reserves wrath for His enemies. The Lord is slow to anger and great in power, and will not at all acquit the wicked." (Nahum 1:1-3)
This short book (three chapters, 47 verses) is dedicated to one thing -- destruction of the blood-thirsty, cruel Assyrian city of Nineveh. An ancient city built by Asshur, a son of Shem, the son of Noah, Nineveh was (From 1100 to 800 BC) the capital city in Assyria. According to Jonah 4:11, it was a huge city. One hundred fifty years earlier, God sent Jonah to preach repentance to this wicked city. She repented at that time, but later returned to her wicked ways. Nahum tells how God was going to punish Nineveh.
This book clearly defines God's awesome power. "Who can stand before His indignation? And who can endure the fierceness of His anger? His fury is poured out like fire, and the rocks are thrown down by Him... But an overflowing flood He will make an utter end of its place, and darkness will pursue His enemies." (Nahum 1:6, 8)
Why did it happen? "Because of the multitude of harlotries of the seductive harlot, the mistress of sorceries, who sells nations through her harlotries and families through her sorceries." (Nahum 3:4) Nineveh's past repentence could not save her from the present trouble. The book of Nahum can help us see that our duty is to remain faithful to God. Any nation that pursues a course of ungodliness and rebellion against God will not escape His condemnation. Each nation must choose. Even a great nation needs to take stock of the course on which it's traveling. God does not change!