Especially under the rules of engagement imposed on them the 'Century Series" aircraft initially specifically designed for intercepting heavy nuclear bombers or delivering tactical nuclear weapons were found to be wanting when they were engaged by the very agile fighters Mikoyan-Gurevich MiG-17 and Shenyang J-6 provided to the VPAF by the USSR and PRC; the Mikoyan-Gurevich MiG-21 while less agile was formidable against the F-4 and traded range for very high performance.