The book of Nahum is addressed to the people of Nineveh, you may have heard of them before?  About 130 years before this, the prophet Jonah also spoke to the inhabitants of that great city calling on them to repent.  The book of Nahum could be known as Nineveh part II or Nineveh the sequel.  Initially after their repentance at the time of Jonah things went fairly well, the Ninevites (Assyrians) enjoyed God’s mercy as he spared not just their people but, the city its self. However, the relationship that they had built up with God began to decline as they benefitted from God’s mercy from their past but neglected to let him into their future.  They did not pass on to the next generation the story of God, they did not teach their children to respect the ways of God.  This encounter that they had with the Almighty God was a great reprieve at the time but they didn’t want it to encroach on their advancement as world leaders.  Cruelty seeped its way back in, victories were gained but at a terrible cost, their national emblem was a lion and it devoured everything around it.  Despite the mercy shown to them they showed none to others and despite the reprieve from destruction they continued to destroy, they poured out on those around them what could only be described as a reign of terror.  While God is merciful he is also just and cannot let such wickedness go unchecked.  

God does not show us his mercy so we can repeat bad behaviour but, so that with his help we can change it.