Joshua 6:12-14. At God's instruction, Joshua has begun a strange siege of Jericho. He rises early in the morning and has the priests take up the ark of the covenant. Then, with seven priests continually blowing on seven trumpets of ram's horns before the ark, preceded by armed men, and followed by the rear guard, the ark and the whole group march around the city. Once the group has circumnavigated the city once, they return to camp. This is done for six days in a row.

Why does God tell Joshua to do this? Well, because it worked! On the seventh day when the people all shouted and the trumpets all blew, the walls of Jericho fell down. Perhaps the marching and the trumpet blasting weakened the walls of the city. But there's another reason why God had them do it, I believe. One can definitely see the psychological effect that this must have had on the people of the city. First it was loud and annoying - the continual loud blowing of trumpets for however long it took to go around the city must have been deafening! Second, it is weird. The noise would have made people come to see what was going on, and they would have seen these strange Jews marching around their city carrying some sort of sacred box, with priests and military men. That must have been very off-putting in its strangeness. Third, this goes on for six days straight. The city dwellers didn't know when it was going to stop or change. They must have thought that this might go on for a very long time. And it was so very annoying and odd! I can see them becoming so preoccupied with this strange siege that they were probably unable to think of or do much else!

Essentially, this is a psyops campaign organized by God against the people of Jericho. Once the walls of the city fell, the people would be easily conquered, having had their minds so addled by six days of this strange procession. And they were, in fact, conquered pretty easily!