John Wesley on Joel 1:6

A nation - An innumerable multitude of locusts and caterpillars, called a nation here, as Solomon calls the conies and the ant, Prov 30:25,26, and perhaps a prognostick of a very numerous and mighty nation, that ere long will invade Judah. Strong - Mighty in power, and undaunted in courage, if you refer it to the Assyrian or Babylonians; if to those vermin, they are, though each weak by itself, yet in those multitudes, strong and irresistible. A great lion - Such waste as lions make, these the locusts do, and the Assyrians will make.