The Lord would gather the armies of the world (literally all the nations) in the valley of Jehoshaphat. This valley is not related to King Jehoshaphat of Judah (1 Kings 22:41). Instead, it means “the valley where the Lord judges.” It is the place where God would decide the fate of the nations (see Joel 3:14).
The sins of the nations included driving God’s covenant people into exile. They also included abusing helpless boys and girls by selling them as slaves (compare Ezekiel 27:13; Revelation 18:13).