At the beginning of the twentieth century, ASW techniques and submarines themselves were primitive. During the First World War, submarines deployed by Imperial Germany proved themselves to be a capable threat to shipping, being capable of striking targets even out in the North Atlantic ocean. Accordingly, multiple nations embarked on research into devising more capable ASW methods, resulting in the introduction of practical depth charges and advances in sonar technology; the adoption of the convoy system also proved to be a decisive tactic. After a lull in progress during the interwar period, the Second World War would see submarine warfare and ASW alike advance rapidly, particularly during the critical Battle of the Atlantic, during which Axis submarines sought to prevent Britain from effectively importing supplies. Techniques such as the Wolfpack achieved initial success, but became increasingly costly as more capable ASW aircraft were introduced. Technologies such as the Naxos radar detector gained only a temporary reprieve until detection apparatus advanced yet again. Intelligence efforts, such as Ultra, had also played a major role in curtailing the submarine threat and guiding ASW efforts towards greater success.