The introduction of the guided missile resulted in a significant shift in anti-aircraft strategy. Although Germany had been desperate to introduce anti-aircraft missile systems, none became operational during World War II. Following several years of post-war development, however, these systems began to mature into viable weapons systems. The US started an upgrade of their defences using the Nike Ajax missile, and soon the larger anti-aircraft guns disappeared. The same thing occurred in the USSR after the introduction of their SA-2 Guideline systems.