The people of Moab believed they could attack Judah without punishment. They treated Judah as if it were just another nation.
This was not true. Judah had been acting like the other nations (see Ezekiel 20:32). However, Judah was different because it had a covenant with God.
God would not finally reject Judah. He would discipline his people, but he would not abandon them forever.
Moab, however, would face judgment. Like Ammon, Moab would be removed from the register of the nations (the official list of recognized nations). It would be left desolate (empty and ruined) forever.