The answer is: Ai

Joshua 7:3 - If Joshua had prayed to God he would have known not to send the men into battle

The defeat at the city of Ai, described in Joshua 7, serves as a pivotal moment in Israel's conquest of Canaan. It illustrates a profound lesson about seeking guidance before making decisions. Joshua, confident after the recent victory at Jericho, made the mistake of relying on his own judgment rather than consulting God. Perhaps he thought the people of Ai would be an easy target, underestimating the situation—and remember this wasn’t a huge city. When he sent only a small group to attack, they were swiftly defeated.

Interestingly, this moment echoes a common theme throughout scripture: when people rely on their wisdom alone, disaster often follows. Think of King Saul, who failed to wait for Samuel's guidance and ended up losing God’s favor. Both events highlight the importance of maintaining a relationship with the divine, which is vital for guidance – showing that it's not just about having confidence but song deeply connected.

After facing loss, Joshua fell prostrate, seeking answers through prayer, which gets quick results. God revealed that unconfessed sin—specifically Achan’s theft of devoted items from Jericho—had led to their defeat. This teaches believers about the communal impacts of individual actions. Sin, as baskets of good and bad fruit across the Bible suggest, doesn’t just affect the individual but can have ripples throughout their tribe or family. Quite a potent illustration, isn't it?