Originally a tail-chasing system, early models saw extensive use during the Vietnam War, but had a low success rate. This modularity allowed for the introduction of newer seekers and rocket motors, including the AIM-9C variant, which used semi-active radar homing and served as the basis of the AGM-122 Sidearm anti-radar missile. Low-level development started in the late 1940s, emerging in the early 1950s as a guidance system for the modular Zuni rocket. The Soviet K-13 (AA-2 'Atoll'), a reverse-engineered copy of the AIM-9B, was also widely adopted. Its latest variants remain standard equipment in most Western-aligned air forces. Entering service with the United States Navy in 1956 and the Air Force in 1964, the AIM-9 is one of the oldest, cheapest, and most successful air-to-air missiles. The AIM-9 Sidewinder ('AIM' for 'Air Intercept Missile') is a short-range air-to-air missile. Aircraft, naval vessels, fixed launchers, and ground vehicles