During the final preparations for Operation Overlord, the Allied invasion of Normandy, Exercise Tiger took place to conduct rigorous training for American and British forces. Slapton Sands in Devon, England, was chosen as the exercise site, simulating the landing conditions of a beachhead assault. On April 28, 1944, while the troops practiced their landing procedures, nine German E-boats launched an audacious nighttime raid. The Opal, the S-138, and other vessels of the German Kriegsmarine penetrated the exercise area undetected, capitalizing on an intelligence failure that resulted in inadequate protection for the exercise fleet.

The E-boats successfully executed torpedo attacks against LSTs (Landing Ship Tanks) carrying troops and supplies. A combination of poor communication among the Allied units and the surprise nature of the assault meant that defensive measures were slow to be mobilized. As a result, many vessels were sunk or heavily damaged, and approximately 946 servicemen lost their lives, most of whom were American troops. This incident not only resulted in a significant loss of life but also prompted a re-examination of operational security protocols for Allied forces as they approached the D-Day invasion.

Following the attack, the Allies took several measures to ensure that such a disaster would not recur. Changes were implemented in how exercises were conducted, and new strategies were adopted to enhance the safety and security of the invasion forces. Additionally, the event highlighted the significant threat posed by German naval forces and raised awareness about the challenges that Allied forces would face on D-Day.