"'See, I am going to rouse them out of the places to which you sold them, and I will return on your own heads what you have done. I will sell your sons and daughters to the people of Judah, and they will sell them to the Sabeans, a nation far away.' The Lord has spoken." NIV translation
The judgment against the nations of Joel's day was that the things that they did to God's people were going to happen to them. This principle was firmly established in God's promise to Abram (see Genesis 12:3). It was true then and it is true today and will be on the last day.
"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." NIV translation
Joel now shifts back to the Day of the Lord and the fact that all nations will prepare for the battle to attack Jerusalem. We see the arrogance of this in the fact that the weak will say "I am strong!" It seems that we are so close to this time as we are told that, if the world would just band together, we can do anything. That is the big lie that Satan will use to convince the nations to attack Jerusalem.
"Bring down your warriors, O Lord! 'Let the nations be roused; let them advance into the Valley of Jehoshaphat, for there I will sit to judge all the nations on every side.'" NIV translation
We see that Satan calls the nations to attack but does not tell them that they will be against the armies that will appear with Jesus. They believe they are coming for a great battle but, as we see here, they are coming to their judgment.
"'Swing the sickle for the harvest is ripe. Come, trample the grapes for the winepress is full and the vats overflow - so great is their wickedness!' Multitudes, multitudes in the valley of decision! For the day of the Lord is near in the valley of decision." NIV translation
When will Jesus put an end to this rebellion? When the nations assemble in the valley of decision.