WASHINGTON — It was only after an American decision to remove nuclear weapons from the Korean peninsula that the scientists behind North Korea’s atomic quest started to make vital breakthroughs.

In 1991, President George H.W. Bush ordered the removal of all nuclear warheads from American bases in South Korea. At one point in the late 1960s, America had 950 warheads deployed to deter the communist North.

North Korea had been working on a nuclear program since the 1960s, and in the mid-1980s, it commissioned a nuclear reactor, built with Chinese help. The 20-megawatt facility near Yongbyon was built to supply electricity, but its plutonium stockpile was also used for weapons research.

Intelligence experts believe that a 70-day shutdown of the Yongbyon plant in 1989 allowed North Korea to remove fuel from the reactor to separate out plutonium.

Subsequent North Korean plans to expand the Yongbyon facility sparked a flurry of international diplomacy. In the 1994 Agreed Framework agreement with America, North Korea stopped construction of a 200-megawatt reactor in Yongbyon and a 700–800 megawatt reactor near Taechon. But secret processing continued, and after several showdowns, North Korea pulled out of the agreement.

The second key boost for North Korea came in the mid-1990s when a representative of the Pakistani scientist A. Q. Khan, honored in his home country as father of its weapons program, offered material and know-how for a fee. Mr. Khan sold North Korea centrifuges to enrich uranium, the material that was used in the Hiroshima bomb. North Korea warned last year that it was determined to emulate America’s first nuclear bomb in Nagasaki, and Washington finally confirmed that it had evidence of a secret North Korean uranium enrichment program in October 2002.

Within two months, Pyongyang had removed seals and monitoring cameras of the nuclear watchdog, the International Atomic Energy Agency, from the mothballed Yongbyon plant.