As events lead up to the final judgment, the Day of the Lord, there will be much war and conflict. The wicked will not go away easily. Joel calls the nations to come forth and do battle before their climactic defeat.
Readers of verse 10 may be reminded of a similar passage in both Isaiah 2:4 and Micah 4:3. In Isaiah and Micah, the text promises that there will be a time of peace when swords will be turned into plowshares and spears into pruning hooks. Joel surely knows this earlier prophecy, but he turns it upside down. He says that a time is coming when instruments of agriculture will be turned back into weapons of war. This is an interesting example of how sometimes the biblical prophet takes what had been understood as a positive word of hope and turns it around into a negative meaning. (Amos does this several times.) A new time requires a new "sermon" on an old word.
The idea that the final destruction of evil will be preceded by a time of turmoil and great battles is common in apocalyptic (end-time) literature. It can be seen in books like Daniel and Revelation. Joel is a move in that direction.
9 Proclaim this among the nations:
stir up the warriors.
Let all the soldiers draw near,
let them come up.
10 Beat your ploughshares into swords,
and your pruning-hooks into spears;
let the weakling say, I am a warrior.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org