Swept wings would reach Mach 2 on the BAC Lightning, and Republic F-105 Thunderchief, built to operate at low level and very high speed primarily for nuclear strike, but with a secondary air-to-air capability.[92] By the late 1960s, the McDonnell F-4 Phantom II, was used in large numbers by air forces influenced by the United States. Variable geometry wings were employed on the American F-111, Grumman F-14 Tomcat and Soviet Mikoyan MiG-27, although the idea would be abandoned for the American SST design. After the 1970s, most newer generation fighters optimized for maneuvering air combat since the USAF F-15 and Soviet Mikoyan MiG-29 have employed relatively short-span fixed wings with relatively large wing area.[citation needed]