Joshua gave clear instructions to stay away from all the riches of the city. However, a man named Achan stole some of the riches of Jericho to keep for himself. Because of this sin, Israel’s army was defeated in their next battle. They lost to a very small army from the town of Ai. This caused great worry in the camp of Israel, and even Joshua was upset by the defeat.
Then Joshua tore his clothes and fell facedown to the ground before the ark of the LORD, remaining there till evening. The elders of Israel did the same and sprinkled dust on their heads. And Joshua said, “… O Lord, what can I say, now that Israel has been routed by its enemies?” … The LORD said to Joshua, “Stand up! What are you doing down on your face? Israel has sinned; they have violated my covenant, which I commanded them to keep. They have taken some of the devoted things; they have stolen, they have lied…. That is why the Israelites cannot stand against their enemies… Joshua 7.6-12
Joshua and the elders found out that Achan took the forbidden things, so he was judged and killed for his disobedience. It was after this that Israel was able to take all of the land that had been promised to Abraham.