Jamieson, Fausset and Brown on Joel 2:20

20. the northern army--The Hebrew expresses that the north in relation to Palestine is not merely the quarter whence the invader comes, but is his native land, "the Northlander"; namely, the Assyrian or Babylonian (compare Jr 1:14, 15; Zep 2:13). The locust's native country is not the north, but the south, the deserts of Arabia, Egypt, and Libya. Assyria and Babylon are the type and forerunner of all Israel's foes (Rome, and the final Antichrist), from whom God will at last deliver His people, as He did from Sennacherib (2Ki 19:35).

face ... hinder part--more applicable to a human army's van and rear, than to locusts. The northern invaders are to be dispersed in every other direction but that from which they had come: "a land barren and desolate," that is, Arabia-Deserta: "the eastern (or front) sea," that is, the Dead Sea: "the utmost (or hinder) sea," that is, the Mediterranean. In front and behind mean east and west; as, in marking the quarters of the world, they faced the east, which was therefore "in front"; the west was behind them; the south was on their right, and the north on their left.

stink--metaphor from locusts, which perish when blown by a storm into the sea or the desert, and emit from their putrefying bodies such a stench as often breeds a pestilence.

because he hath done great things--that is, because the invader hath haughtily magnified himself in his doings. Compare as to Sennacherib, 2Ki 19:11-13, 22, 28. This is quite inapplicable to the locusts, who merely seek food, not self-glorification, in invading a country.