Why must Moab be made drunk?  

There follows what appears to be a command from the Lord to those who are bringing destruction on Moab: Make him drunk. This echoes the original context for these oracles against the nations, including Moab, where the Lord told Jeremiah to take this cup of wrath, and make all the nations to whom I send you drink it (Jeremiah 25:15). The cup of wine is a common metaphor for the Lord’s judgment and the drunkenness of those who are forced to drink it describes the overwhelming impact that the Lord's judgment has.

Two reasons for the Lord’s judgment are also given. First, Because he magnified himself against the Lord. This same accusation is made in Jeremiah 48:42. The most likely reference here is the conference in Jerusalem in 594 BC mentioned in Jeremiah 27:3. Moab was part of that meeting as nations gathered to plot rebellion against Babylon. Jeremiah understands that Nebuchadnezzar was the Lord’s servant, and so to resist Babylon was to magnify yourself against the Lord and meant facing the Lord’s anger.¹

The second reason for the Lord’s judgment of Moab is her attitude towards Israel. Was not Israel a derision to you? It would seem that Moab had mistreated Israel by attempting to take territory from her: Was he found among thieves, that whenever you spoke of him you wagged your head? Compare also what the prophet Zephaniah says about Moab’s treatment of Israel (Zephaniah 2:8–11).²In the end, the Lord avenges the mistreatment of his people, even if his people themselves face judgment for their own behaviour.