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Could the German U Boat Type XXI have decisively altered the course of the Second World War at sea if they had been available in greater numbers? - Quora
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If the Type XXI Uboat was manufactured earlier in WW2, would it have a profound impact on the course of WW2? - Quora
This Map Shows the Full Extent of the Devastation Wrought by U-Boats in World War I, History
The WW1 Q-Ships Were Heavily-Armed Decoy Vessels, Designed To Lure Unsuspecting U-Boats Into A Trap