The occasion for the book seems to be a devastating locust plague. The imagery of the locusts is conflated, however, with images of invading armies. Joel calls for YHWH's people to lament and repent.
Rend your heart
and not your garments.
Return to the Lord your God,
for he is gracious and compassionate,
slow to anger and abounding in love,
and he relents from sending calamity.
Who knows? He may turn and relent
and leave behind a blessing —
grain offerings and drink offerings
for the Lord your God.
(Joel 2:13-14, NIV)
These verses are part of a common reading for Ash Wednesday.
In response to Israel's repentance, YHWH restores his people's prosperity.
The remainder of the book is occupied with "the Day of the Lord." For Judah this will be a time of blessing.
I will pour out my Spirit on all people.
Your sons and daughters will prophesy,
your old men will dream dreams,
your young men will see visions.
Even on my servants, both men and women,
I will pour out my Spirit in those days.
(Joel 2:28, NIV)
The Apostle Peter quotes this passage in his Pentecost speech in Acts 2. He proclaims that the outpouring of the Holy Spirit on that day fulfills Joel's words.
Other nations will not fare so well as Judah on the Day of the Lord.
Proclaim this among the nations:
Prepare for war!
Rouse the warriors!
Let all the fighting men draw near and attack.
Beat your plowshares into swords
and your pruning hooks into spears.
Let the weakling say,
"I am strong!"
Come quickly, all you nations from every side,
and assemble there.
Don't miss the way that verse 10 reverses the words of Isaiah 2:4 and Micah 4:3 about beating swords into plowshares and spears into pruning hooks. Joel, apparently, was well-versed in Hebrew Scripture.
Next: Amos 1-5