In these first few verses Joel refers back to what he spoke on in chapter 2 about the day of the Lord at hand and how He will would judge the nations around Judah (Israel). The nations are being judged because of how they treated God's chosen. They had divided up the land, casted lots against them, and turned them into slaves.
I love these verses because God is defending His people with a vengeance. He list in verse 4 the actual cities that have wronged His people and then goes into detail in the next verses about He will show judgement. The basic message that is portrayed is that if these people didn't repent and recompense what they had taken God was going to do worse on them. I am thankful that I serve a just God.
In verse 9 Joel is proclaiming to the people to be prepared for war. When things like this are proclaimed from God's messengers the people are to prepare by getting offerings and lifting up prayers. In verse 10, every available person, even the weak, along with anything that can be used as a weapon are called to action. All the people of the nations were then called to go to the valley of Jehoshaphat but while they were thinking it was for battle it really was so that they would all be together for judgement. Based on the how Joel describes all of these events would tell us that he was off in the distance watching all of the signs unfold. God wanted the people of Judah to see that He would protect and take care of them.
These verses are just reiterating the fact that God will replenish the land of Judah and they will be taken care of forever. However Egypt and Edom will be quite different, becoming like wilderness and a waste. In the last verse it concludes with the promise from God that He will avenge their blood which have yet to be avenged.