According to the Bible, the walls of Jericho collapsed because God had given the city to the Israelites and instructed them to march around the city for six days with the Ark of the Covenant. On the seventh day, the Israelites were to march around the city seven times, blow their trumpets, and shout. When they did so, the walls of Jericho collapsed, allowing the Israelites to conquer the city. Therefore, it was the divine intervention of God that caused the walls to collapse, not because they were too fragile or due to any decree made by Joshua.