Nahum is a prophet to Judah, because Israel has already been destroyed by the Assyrians. He is prophesying against Nineveh, the capitol of Assyria. Jonah had also preached to Nineveh and they had turned to God, but that turning to God was obviously short-lived. Since then, Assyria has invaded and destroyed Israel. Like Obadiah鈥檚 message to Edom, God is letting Assyria know that just because God has used them to discipline Israel doesn鈥檛 mean God is going to overlook their wrongdoing.
God鈥檚 message of judgment was to both those who had greater exposure to God, like Israel and Judah, and those who had less, like Assyria. There are none who should be outside our reach in bringing people to Christ; the same judgment awaits everyone, and the same salvation is available to all.