Daring U-Boat commanders carried out torpedo attacks within the defensive screen, and when several submarines attacked at once, the defenders had little chance of striking back.
Submarine warfare had some impact in the First World War, but became vastly more significant in WWII as the German U-boat packs aimed to blockade Europe. Merchant ships took to sailing in large convoys, protected by screens of destroyers and corvettes armed with depth charges and sonar. Daring U-Boat commanders carried out torpedo attacks within the defensive screen, and when several submarines attacked at once, the defenders had little chance of striking back.