Nahum receives an oracle, a vision, from the Lord. This, again, is the normal mode of prophesy, Jonah and Micah, the past two days, are not reported as a vision (Micah may be, but Jonah most likely is not due to his 'transitional' nature as a classic to writing prophet role). But vision is the normal mode.
So, if in Obadiah "Edom is toast", then in Nahum, Nineveh is the fried egg that completes the spread. The fate of Nineveh due to their treatment of God's people and arroagance is sure. They will be destroyed.
Why is the Lord going to destroy Nineveh so ruthlessly after two days ago we read of the lesson He taught Jonah (and Israel) about compassion with Nineveh as the object lesson? First, this historically is a different Nineveh that repented in the presence of Jonah, since about or over 100 years have passed. But more importantly, the line of David is in question. The Lord is not 'flip-flopping' on His judgements. It is important and helpful to remember that when we read the writing prophets, the Lord is preserving His people, yes, because He loves them, but more importantly because they carry the line of David that will bring the Messiah - the King and redeemer of the world. Sometimes the Lord's word through His prophets can seem very strange and at times contradictory, but if the preservation of the line of David is kept in mind many prophesies will start to make more sense (Most importantly when the Lord seemingly works against His people, due to their actions, idolatry, or simply apathy, that can thwart the succession of the line of David. Or, when a nation for which he previously had compassion or used in His plan, like Nineveh, suddenly is the object of His wrath).
1) The Lord judges all nations and controls history. We have seen now in Micah and Nahum a prophesy against a people, namely Israel's leaders and Nineveh, that became reality soon after. The nationality of the country matters not to the Lord when it concerns His plan and arrogance against His mission to establish His Kingdom.
2) The Lord will protect His people. Nineveh has become an arrogant city that oppresses Israel, and though the Lord used Assyria to discipline His people for their waywardness, He will not allow Assyria to wipe Israel off the map. The love of His devotion to His people is strong especially, again, in view of the line of David.
3) The Lord's plan of the restoration of Israel is still in view. The Lord has not forgotten His mission and plan to establish His Kingdom on earth among all people (the meaning of "keeping your feasts" is remembering the Lord's promises; Neh. 1:15).
"Behold, upon the mountains, the feet of him who brings good news, who publishes peace! Keep your feasts, O Judah; for never again shall the worthless pass through you; he is utterly cut off." Neh. 1:15