Joel 2:20

The prophet now revealed that this invader would come from the North. Both Assyria and Babylon, as well as all other eastern invaders, entered Israel from the north because of the impassability of the Arabian Desert to Israel’s east.

"If ’the northerner’ is yet future (eschatological), the army is possibly the army in Joel 3:9; Joel 3:12; Daniel 11:40; and Zechariah 14:2." [Note: Chisholm, "Joel," p. 1419.]

Instead of leading this army against Jerusalem (Joel 2:11), the Lord would drive it from Judah. He would drive its soldiers into a parched and desolate land (Arabia?) and into the eastern (Dead) sea and the western (Mediterranean) sea (cf. Daniel 11:45). In other words, He would turn against them rather than leading them and scatter them rather than uniting them against Jerusalem. The smell of the dead carcasses of the many soldiers would fill the air because they had done many great things. In short, they had tried to overthrow God’s people (cf. the Egyptians drowning in the Red Sea). Masses of dead locusts also smell terrible, especially after dying in the sea and then being washed ashore. [Note: Driver, pp. 62-63; Smith, 2:441.]