Start of Cold War: US & Soviet navies need to stake claims in their respective national security constructs. Soviets can’t compete with West’s navies symmetrically, made same decision the Germans had in both world wars: make a submarine force able to cut Atlantic supply lines. US submarine service recognized where Soviets were heading, evolved existing subs to an anti-submarine role. Both used the most technologically sophisticated sub in the world as a design template, the German Type XXI U-boat. Type XXI design requirements sprung from Battle for the North Atlantic, features attractive to Superpowers. Soviet Whisky class & its combat roll. US “GUPPY” adaptation of WW II Fleet boats to hunter-killers. Injections of Type XXI technology. Capt. Mark McDonagh, USN/ret., former submarine officer with 12 years’ Naval War College experience, on the National Security Decision Making Game staff.