threshold states—states with the technical capability to develop nuclear weapons and the security motivations to seriously contemplate such an option. The other states were Israel, South Africa, South Korea, and Yugoslavia. U.S. intelligence also believed that Taiwan had designed devices suitable for nuclear testing. In 1980 the Taiwanese contracted for 4,000 tons of uranium metal from South Africa although it is not known how much of this order was ever delivered. The secret nuclear weapon program was revealed after the 1987 Lieyu massacre, when Colonel Chang Hsien-yi Deputy Director of Nuclear Research at INER, who was secretly working for the CIA, defected to the U.S. in December 1987 and produced a cache of incriminating documents. General Hau Pei-tsun claimed that scientists in Taiwan had already produced a controlled nuclear reaction. Under pressure from the U.S., the program was halted. A study into the secret program concluded that at the time of Chang's defection, Taiwan was one or two years away from being able to complete a deliverable weapon. Chang claims that Madame Chiang Kai Shek and military officials loyal to her had expedited the development of nuclear weapons and even set up a parallel chain of command to further their agenda. In 1987 the warhead design had a diameter of 60–70 cm with casing and a weight of 900 kg meaning that further miniaturization would have been needed to optimize the weapon for delivery. Delivery systems The Sky Horse ballistic missile system was developed in the late 1970s and early 1980s before a combination of pressure from American President Ronald Reagan and internal competition from anti-ballistic missile development programs ended the program in 1982. Faced with an inability to field their desired delivery system planners turned to alternatives. The secondary delivery vehicle was the AIDC F-CK-1 Ching-kuo, then in development. Weapons designers at CSIST (now NCSIST) were instructed to design the nuclear weapon to fit within the dimensions of the F-CK-1's centerline drop tank (~50 cm diameter). Modifications to the aircraft would have been required for it to obtain the desired 1,000 km range (especially without the centerline drop tank available). The F-CK-1 would not have been able to fulfill the nuclear strike mission until mid-1989 at the very earliest. Early designs called for an unpowered gravity bomb. Current status Since the end of the nuclear weapons program the “Nuclear Card” has played an important part in Taiwan's relationship with