Military leaders took away the lesson that underestimating the enemy can lead to disaster, making the correct answer Underestimating the enemy can lead to disaster.

The Battle of the Bulge began with the Allies underestimating the strength and capabilities of the German forces, which led to significant initial setbacks. The surprise attack and the severity of the German offensive caught Allied commanders off guard, emphasizing the importance of thorough intelligence and respect for enemy capabilities. This experience reinforced the need for military leaders to remain vigilant and cautious, even when previous victories might suggest a lack of immediate threat. The battle served as a stark reminder that complacency and overconfidence can have dire consequences in warfare.