Before God’s final judgment is fully executed, there will be great conflict among the nations.
As events lead up to the final judgment, the The Day of the Lord, in prophetic writing, is the day of judgment when God will intervene directly in world affairs. As described in Zephaniah, for instance, God will sweep everything away. In Matthew's gospel God is described as gathering the elect on the day... More, 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, son of Amoz, who prophesied in Jerusalem, is included among the prophets of the eighth century B.C.E. (along with Amos, Hosea, and Micah)--preachers who boldly proclaimed God's word of judgment against the economic, social, and religious disorders of their time. More 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 is the gift, inspired by God, of speaking and interpreting the divine will. Prophets such as Amos, Isaiah, and Ezekiel spoke words of judgment and comfort to the people of Israel on behalf of God. More, 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. (Prophet to the northern kingdom who condemned Israel's oppression of the poor, calling for justice to "roll down like waters." More 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 An interpreter of dreams who was delivered from the lions' den. More and Revelation. Joel is a move in that direction.