President Bill Clinton took the podium on October 18, 1994, with a speech that reads like a sigh of relief—the announcement of a landmark nuclear agreement between the United States and North Korea. “This agreement is good for the United States, good for our allies, and good for the safety of the entire world,” he assured the nation. Called the Agreed Framework, it was designed to put the brakes on North Korea’s nuclear program, and it promised to put an end to years of increasing nuclear tension, including a near war, to a halt.

“This agreement represents the first step on the road to a nuclear-free Korean Peninsula,” Clinton said. “It does not rely on trust.” In exchange for North Korea ending its nuclear weapons program, the United States agreed to normalize relations with the nation—and both agreed to pursue “formal assurances” not to use nukes against one another.

The agreement—forged against all odds in an environment of fear and worry—seemed bulletproof. So why did it fail just a few years later? The reasons why are rooted in behind-the-scenes negotiations and international mistrust.

North Korea had been preparing for nuclear war since the Cold War, when the USSR began to train North Korean scientists to build nuclear weapons. As part of the Communist bloc, North Korea was closely aligned with the USSR, and Moscow provided the technology, training and even geological surveys that helped North Korea locate local deposits of graphite and uranium ore that could be used to create nuclear weapons.

According to Derek Bolton, who works with the national security think tank American Security Project, North Korea was well on its way to a nuclear weapons program by the 1960s, and had conducted successful experiments with fission, the underlying chemical phenomenon that can cause a nuclear reaction, under the supervision of the USSR as early as 1963.

Over the years, North Korea tried to find more support for its nuclear program, includingengaging South Korea in talks about whether the two countries should develop a joint nuclear weapon in secret. (South Korea declined.) But it took until the 1980s for the world to realize that North Korea might be serious about building nukes—and to recognize that it might be closer to nuclear weapons than previously thought.

Despite its apparent commitment to developing nuclear weapons, North Korean leader Kim Il Sung did ratify the Treaty on the Non-Proliferation of Nuclear Weapons in 1985. The international treaty, which was designed to prevent the spread of nuclear weapons, had been in force since 1970, but North Korea had lagged behind other nations like the United States. Now that North Korea was on board, though, it also began mining uranium and producing plutonium—both critical to the production of nuclear weapons—and creating nuclear reactors during the 1980s. Then, in 1989, the Soviet Union fell, leaving North Korea increasingly isolated.

“With the collapse of the Soviet Union, North Korea lost its main protector,” Georgetown University professor Keir Leiber toldVox’s Zack Beauchamp. “What does it have that can counter conventional US power? The answer is obvious: nuclear weapons.”

That same year, the U.S. discovered Kim Il Sung’s covert nuclear program using satellite imagery, and North Korea kept developing weapons even afteragreeing with South Korea not to test or manufacture nukes. As a result, the International Atomic Energy Agency, an autonomous nuclear oversight organization that reports directly to the United Nations, asked to conduct inspections of North Korean nuclear sites in 1992 and 1993. North Korea refused, and threatened to back out of the Nuclear Non-Proliferation Treaty.

This represented a double crisis for then-President Clinton. Republicans in Congress pressured him not to negotiate with North Korea, but the international community and Democrats argued that engagement was the only solution. Meanwhile, North Korea escalated its rhetoric, telling the United States that North Korea would turn Seoul into “a sea of flames” if the U.S. pursued sanctions through the United Nations.

The U.S. considered military intervention, but also sent Jimmy Carter to Pyongyang to meet with Kim Il Sung. Carter convinced Kim to start nuclear talks—but the day negotiations were supposed to begin, Kim died. He was succeeded by his son, Kim Jong Il, the very man who founded the most controversial nuclear complex in North Korea, a facility in Yongbyon.

Things looked grim, but Clinton became increasingly convinced that direct negotiations were the only way. However, American negotiators doubted from the start that diplomacy would work. “The initial contacts were to test the proposition that we could address their security concerns by getting them to give up their nuclear weapons,” Robert Gallucci, the chief negotiator, toldBeyond Parallel in a 2016 interview. “It was not so much of a conviction on anybody’s part…It was possibly true, and worth testing.”

For 16 months, Gallucci and his team conducted intense negotiations with North Korea. The countries locked horns on what it would take for North Korea to stop producing nukes. Finally, they came to an agreement—the Agreed Framework.

Just four pages long, the agreement said that North Korea would shut down its main nuclear reactor in Yongbyon, abandon two others, and seal fuel that could potentially be used to create a nuclear weapon. In exchange, the U.S. would provide oil to make up for the fuel lost from the dismantled plants and would build two new “light fuel” plants from which it would be harder to extract nuclear materials. If North Korea did try to get fuel out of the new plants, it would be easy for nuclear watchdogs to identify—and hard to hide. In addition, the agreement promised that the U.S. would lift economic sanctions and its diplomatic freeze on North Korea and agree that it would not use nuclear weapons of its own on North Korea.

On the surface, it looked like the U.S. was offering huge concessions to North Korea in exchange for few assurances. But behind the scenes, the Clinton administration thought that North Korea was on the verge of collapse and likely wouldn’t last long enough for the U.S. to build the agreed-upon reactors. In North Korea, the agreement wasn’t taken seriously. Isolated, impoverished and headed by a leader who believed nuclear power would give the country power on the international stage, North Korea had little motivation to give up its program.

Clinton knew the agreement would be hugely controversial—so he structured it in a way that ensured it wouldn’t have to be ratified by the Senate. Republicans were infuriated. And shortly after the agreement was signed, the Republicans won control of Congress. They grilled Gallucci. “It was pretty harsh,” he told PBS in 2003. “We did not get ticker tape parades, as it turned out.” Congress made it clear that they would not agree to actually fund the implementation of the project or sanction formal peace agreements between the two countries.

Meanwhile, North Korea continued producing uranium. Kim Jong Il, it turned out, had used potential nukes as a bargaining chip—even though he had no intention of stopping the program. Despite promising initial results, North Korea began flouting the agreement more and more. North Korea ignored warnings that the agreement was in jeopardy and soon intelligence agencies realized it possessed much more advanced nuclear tech than the U.S. had suspected.

At first it seemed like George W. Bush, who took office in 2001, might continue Clinton-era diplomatic policies toward North Korea. But then things fell apart. Bush’s diplomats stopped sending fuel shipments; North Korea complained bitterly that the promised nuclear reactors had never been built. And when the September 11 terrorist attacks happened, it pushed American diplomacy in other directions—and Bush mentioned North Korea as one of the three countries in his “Axis of Evil” State of the Union Speech in 2002.

Soon, relations between the two countries were openly tense, if not hostile. North Korea dropped out of the Nuclear Non-Proliferation Treaty in 2003. By 2006, it had conducted its first nuclear test—an underground delivery that may have been a fizzle, or unsuccessful explosion. And though Bill Clinton himself headed to North Korea to successfully negotiate the release of two American hostages in 2009, it was too late to halt North Korea’s march toward nukes.

Though the United States continues to try to seek solutions to North Korea’s potential nukes, including the potential of talks between President Donald Trump and Kim Jong Un, Clinton’s vision of an end to nuclear proliferation on the Korean Peninsula now seems more like a mirage.