With the development of guided missiles, particularly surface-to-air missiles, plans for dedicated escort fighters designed to escort nuclear bombers gradually faded from the scene. Missile technology meant that interceptors would rarely engage bombers directly, if ever, and the escorts could do little against missiles. At the same time, the advancement of land and submarine-based ballistic missiles relegated bombers to a lower importance — they became just a single element of the nuclear triad in the US, and largely ignored entirely in the USSR. Furthermore, with the concept of mutually assured destruction high on the political agenda throughout the Cold War, a nuclear exchange became ever less likely, leaving existing fighter designs more than adequate for their protection in the wars being fought. In Vietnam for instance, F-4 Phantom IIs and sometimes F-8 Crusaders escorted the American bombers such as B-52 Stratofortresses, F-105 Thunderchiefs and A-4 Skyhawks. In some cases the missions of F-4 were "mixed", when some F-4 were equipped with bombs, and some F-4 acted as escorts (similar cases occurred with F-8).