Sonar is a technology that helps in detecting the submarine sounds in the water. This technology was a result of British initiative for use against German submarines during the First World War. Radar is a technology that employs the use of radio waves for the detection of objects above the land and on sea surface. Radar was developed for the detection of aircraft. The development of these technologies during the Second World War enhanced the capacity of the Allies to detect and destroy German submarines, with any assistance that the snorkel could provide being too late. (Part 6: World War Two Subs) Conclusion:

The Germans showed the world how potent submarines could be in the theatre of war. Their cost effectiveness along with their scope of wide variety of actions and with the capability to operate at long distances from their bases had the effect of causing a lot of focus on their development to their potential. Just the possibility of the presence of a submarine is enough to tie down large resources of the adversary and that too without the need to fire a single shot. In the event of detection, it is quite capable of evading its hunter and thus making it all the more potent. During the days of the Cold War with the piling up of nuclear weapons on both sides, the threat of a nuclear war always loomed. Yet not a single nuclear bomb was used perhaps fearing the nuclear submarines...

Therefore submarines are going to be part of the arsenal of many a nation with both attacking and defensive roles. They have the potential to rule the seas just as aircraft rule the skies and the combination of air superiority with a superior submarine arm is a winning proposition in any global war.