Theme of The Book of Joel

Main Theme - The Kingdom of Judah. Joel 2:1-3 - Blow ye the trumpet in Zion, and sound an alarm in my holy mountain: let all the inhabitants of the land tremble: for the day of the LORD cometh, for [it is] nigh at hand; A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread upon the mountains: a great people and a strong; there hath not been ever the like, neither shall be any more after it, [even] to the years of many generations. A fire devoureth before them; and behind them a flame burneth: the land [is] as the garden of Eden before them, and behind them a desolate wilderness; yea, and nothing shall escape them.

Link: https://bible-history.com/old-testament/bookofjoel