''You couldn't get this stuff from intercepts and you couldn't get it from overhead,'' he added, referring to covert electronic-eavesdropping and satellite reconnaissance systems. ''You had to get it from a human source. And you had to use it very carefully.''

In December 1987, as the secret program was gaining steam, Colonel Chang defected to the United States, with the C.I.A.'s assistance, smuggling reams of documents out of Taiwan: damning evidence of the progress Taiwan had made toward building a bomb. State Department officials pressured Taiwan, which agreed to halt the program.

''This was a case where they actually did something right,'' Mr. Lilley said, referring to the United States intelligence and diplomatic communities. ''They got the guy out. They got the documentation. And they confronted the Taiwanese.''

Taiwan's official position ever since has been that it will not use its scientific and technical expertise to build nuclear arms.

The Republic of China on Taiwan was established by Chiang Kai-shek, the leader of the Chinese Nationalists, who fled with two million followers after Mao's Communist forces took control of the mainland in 1949. China regards Taiwan as a ''renegade province,'' and from time to time has threatened to attack if Taiwan develops a nuclear bomb.

These tensions rise and fall; after China test-fired missiles into the waters off Taiwan's coast in 1995, Taiwan's President, Lee Teng-hui, told the National Assembly that Taiwan should consider reviving its nuclear weapons program. Days later, he said that Taiwan would ''definitely not'' resume work on a bomb.

The article in the Bulletin of the Atomic Scientists, written by David Albright, president of the Institute for Science and International Security in Washington, and Corey Gay, a policy analyst at the institute, is the most thorough study available on Taiwan's nuclear weapons program.