Final answer:
The Allied Powers implemented the convoy system and laid mines in the North Sea to defend against U-boats. American naval escorts and the establishment of a convoy system significantly diminished the U-boat threat.
Step-by-step explanation:
Once the US joined the Allied Powers during World War I, they implemented several strategies to defend against U-boats. One of the key tactics was the convoy system, where merchant vessels traveled in large groups with a guard of circling destroyers and cruisers as protection. This helped to deter U-boat attacks and greatly reduced shipping losses.
In addition, the Allied Powers laid mines in the North Sea from Scotland to Norway, which restricted the movement of German U-boats and made it harder for them to reach the Atlantic Ocean. The presence of American naval escorts in the British surface fleet also played a crucial role in countering the U-boat threat.