SINGAPORE, U.S. President Donald Trump and North Korean leader Kim Jong-un are steeling themselves for a historic showdown over the communist regime's denuclearization, a goal that has been elusive for more than two decades.

Past negotiations, including the six-party aid-for-disarmament talks, have failed to remove the North's nuclear threats due to deep-seated distrust, a security dilemma, domestic political obstacles and geopolitical dynamics.

The Trump-Kim summit on Tuesday is expected to be a touchstone for whether the leaders can overcome such past hurdles and clinch a deal that many hope will help establish a lasting peace on the Korean Peninsula.

N. Korea's nuclear ambitions

North Korea's ambitions for nuclear armament began in the late 1950s, when the U.S. military deployed nuclear arms to the South, which Pyongyang thought undermined the security balance on the peninsula.

The North started to nurture nuclear scientists by sending them to a research institute in Dubna, Moscow Oblast, in 1956. Its research cooperation with the Soviet Union further increased under a 1959 accord for the "peaceful use" of atomic energy.

With assistance from Moscow, the North constructed an atomic research center in Yongbyon, north of Pyongyang, in the early 1960s. After China succeeded in a nuclear test in 1964, the North also tried to secure technological support from its ally but to no avail.

From the mid-1960s to the early 1970s, Moscow helped the North construct a 2-megawatt research reactor, called the IRT-2000, and related facilities. Since the facility was part of a research project, the North signed a partial safeguard agreement with the International Atomic Energy Agency (IAEA) in 1977, which put it under a regular inspection regime.

The North embarked on a more serious nuclear project in 1979, when it started to construct a 5-megawatt electrical graphite-moderated reactor capable of producing spent fuel rods, which if reprocessed, can produce weapons-grade plutonium.

The new reactor began operating in 1986. Washington already learned about the reactor construction through satellite imagery that revealed Pyongyang's construction in 1984 of a reactor cooling tower.

In the mid-1980s, the North also planned to build 50-megawatt and 200-megawatt reactors with Moscow's help. But the project fell apart amid nonproliferation cooperation between the U.S. and the Soviet Union.

Washington requested that Moscow persuade Pyongyang to halt its proliferation activities. In December 1985, the North joined the Nuclear Non-Proliferation Treaty (NPT) with Moscow's pledge to provide support for a light-water reactor.

The North's nuclear program developed into a global issue as Pyongyang refused to sign a full-scope safeguard agreement with the IAEA. A signatory to the treaty should ink the agreement within the first 1 1/2 years after its entry into the NPT.

The North has since used its nuclear program as a bargaining chip, issuing a catalogue of demands, such as the withdrawal of U.S. nuclear arms from the peninsula and a ban on the entry of any foreign nuclear-capable weapons here.

The winding down of the Cold War from 1989-1991 did not make Pyongyang feel any safer, as it faced severe isolation amid Seoul's "Northern Policy" initiative to establish diplomatic ties with China, the Soviet Union and other members of the then communist bloc.

Seoul argued that the initiative was aimed at inducing Pyongyang to open up to the outside world and foster peace on the peninsula. But it dealt a serious diplomatic blow to Pyongyang, which thought the communist bloc would regard it as Korea's only legitimate government.

As part of efforts to address isolation and the security imbalance caused by the military superiority of the South Korea-U.S. alliance, the North stepped up its program to develop nuclear warheads, as well as an assortment of ballistic missiles.

First nuclear crisis and Geneva accord

The North's first nuclear crisis erupted in March 1993, when it announced its decision to withdraw from the NPT in an angry response to the IAEA's request for an intrusive inspection into its nuclear facilities, including two unreported nuclear waste storage sites.

During its earlier inspections, the IAEA found discrepancies between the North's initial declaration and its findings. To address them, it demanded access to additional information, including about the controversial storage sites.

The North rejected the demand, calling it an "infringement" on its "supreme interests." It, in addition, claimed that the U.N. nuclear agency had lost impartiality under the influence of Washington.

Further infuriating Pyongyang was the resumption of the annual South Korea-U.S.' Team Spirit military exercise, which it criticized as a rehearsal for a nuclear war despite the allies' claim that it was defensive in nature.

In 1992, the allies skipped the drills after the North's agreement to accept a safeguard agreement and an IAEA inspection program. But the exercise resumed on March 9, 1993, due to Pyongyang's refusal to embrace a more rigorous inspection program involving the two Koreas.

Three days after the start of the exercise, the North declared its pullout from the NPT, challenging the U.S. efforts to maintain the NPT. The declaration came ahead of a crucial meeting in 1995, where signatories were set to discuss whether to extend, revise or abolish the NPT.

To get the North back in the NPT and stop its proliferation activities, Washington held three rounds of talks with Pyongyang -- the first in New York and the remainder in Geneva, Switzerland -- between June 1993 and October 1994.

After the New York meeting in 1993, the two sides issued a joint statement that included Pyongyang's decision to "unilaterally" suspend the effectuation of its NPT withdrawal as long as it considered the measure necessary, and Washington's assurance against the threat and use of force, including nuclear weapons.

Despite the statement, tensions between the North and the IAEA continued, as the agency sought to strengthen its inspections of the North's nuclear program, with the North rejecting any full-scale inspection regime.

From the second round of the talks, Washington focused on a freeze on the North's nuclear program, while Pyongyang sought to normalize the bilateral relationship and secure energy aid in return for the freeze.

In May 1994, tensions spiked as the North began to remove some 8,000 spent fuel rods from its graphite-moderated reactor -- a step that thwarted efforts to verify Pyongyang's past plutonium production.

It was a reaction to an earlier U.N. Security Council statement that called for the North's cooperation with the IAEA inspection and warned of "additional consideration" in an allusion to punitive measures.

The work on the fuel rods prompted the talk of economic sanctions, which Pyongyang argued would amount to a "declaration of war." The U.S. on its part considered military options, including a surgical strike, to tackle the nuclear standoff.

To de-escalate tensions, former U.S. President Jimmy Carter flew to Pyongyang on a "private" mission in June. He held talks with then North Korean leader Kim Il-sung, who expressed his will to freeze the nuclear program.

Kim died suddenly of a heart attack in July, but Washington and Pyongyang held their third round of talks in two stages between August and October. The last meeting produced the Agreed Framework, better known as the Geneva accord.

Under the framework, the North agreed to freeze all graphite-moderated reactors and remain in the NPT, while the U.S. agreed to provide heavy oil, make arrangements to build two 1,000-megawatt light-water reactors and move toward the normalization of political and economic ties.

Soon after the agreement, it faced a political hurdle in Washington.

In the November midterm elections, the Republican Party, which was skeptical of engagement with the North, secured control of both the House and Senate in an electoral "revolution." Some party members decried the Geneva accord as a "reward for nuclear blackmail."

Seoul, Washington and Tokyo also faced a conflict over how to share the cost for the $4.5 billion project to construct the light-water reactors, while the Asian financial crisis in 1997-98 raised the prospect of funding problems. Seoul shouldered some 70 percent of the costs to help improve inter-Korean ties, with Tokyo financing some 20 percent.

In 1998, hard-liners in Washington raised suspicions that Pyongyang had run an underground nuclear facility in Kumchang-ri. The suspicion was later cleared after an on-site inspection of the facility found no nuclear activity.

Tensions flared anew as the North test-launched a long-range Taepodong-1 missile in August 1998. The provocation reinforced the narrative in Washington that the North had not renounced its nuclear ambitions yet.

Pyongyang's distrust toward Washington further deepened following a change of the U.S. government in 2001. During his State of the Union address in 2002, then-President George W. Bush called the North part of the "axis of evil," along with Iran and Iraq.

The Geneva accord virtually collapsed after the U.S. Department of State claimed in October 2002 that the North had admitted during a visit to Pyongyang by James Kelly, then assistant secretary of state, that it had a covert uranium enrichment program.

About a month later, Washington stopped enforcing the accord by halting its shipment of heavy oil. Pyongyang then resumed its nuclear program and announced its decision to pull out of the NPT in January 2003.

Second nuclear crisis and six-party talks

Despite the North's denial, the suspicions about its clandestine uranium enrichment program sparked the second nuclear crisis.

With the Iraq War topping its foreign policy agenda in 2003, Washington sought strategic cooperation with Beijing in tackling the nuclear quandary, which led to the creation of the six-party talks, involving the two Koreas, the U.S., China, Japan and Russia.

Months before the launch of the multilateral talks, China brokered trilateral talks with the U.S. and the North in April 2003. There, Pyongyang made a set of demands, such as a nonaggression pact with Washington, the normalization of relations and the provision of light-water reactors. But they were not accepted.

The initial round of the six-party talks took place in Beijing in August 2003. Little progress was made as Washington demanded a "complete, verifiable and irreversible dismantlement" (CVID) of the nuclear program, while Pyongyang stuck to an incremental denuclearization process with rewards given for each step it takes.

At the following talks in 2004, the U.S. and the North continued to clash over the demand for the CVID, which the North decried as a move to encroach upon its right to "peaceful" nuclear activities. Pyongyang also insisted that it would freeze its nuclear program only after the resumption of energy aid.

Later that year, friction between Washington and Pyongyang escalated due to the South Korea-U.S. military drills in August and the congressional passage of a North Korea human rights act in October.

The mood grew more acrimonious in January 2005, when then new Secretary of State Condoleezza Rice lambasted Pyongyang as an "outpost of tyranny." The North later spewed out invectives against both Bush and Rice.

The following month, the North declared that it had built nuclear arms as a "self-defense" measure and that it would boycott the six-party talks indefinitely.

Later that year, Seoul made diplomatic efforts to reduce the tensions between the two sides and revive the multilateral dialogue. Washington and Pyongyang then moved toward a compromise at the six-party talks that resulted in the Sept. 19 joint statement.

The statement entailed Pyongyang's commitment to abandon "all nuclear weapons and existing nuclear programs," and other parties' agreement to discuss the provision of a light-water reactor at an "appropriate time." It also mentioned the principle of "commitment for commitment, action for action."

The statement, however, faltered before the ink dried. The U.S. and Japan demanded the North first dismantle its nuclear program before any light-water reactor assistance, while the North wanted to secure the reactor first.

Washington's financial sanction on the North, which was issued shortly after the statement, further infuriated Pyongyang. The U.S. government designated the Macao-based Banco Delta Asia (BDA) bank as a primary money laundering concern, a decision that not only froze North Korean money in the bank but also scared away other institutions from dealing with Pyongyang.

The BDA sanction indicated that Washington was moving to target not only Pyongyang's nuclear program but also a wider range of its illicit activities, including its counterfeiting of U.S. dollars, drug dealings and human rights abuses.

In an angry reaction, Pyongyang test-fired a barrage of ballistic missiles in July 2006. Two months later ahead of the U.S. midterm elections, the reclusive state carried out its first underground nuclear test.

The escalating crisis highlighted the need for the resumption of the multilateral talks. After the nuclear experiment, the U.S. was seen resuming dialogue with the North, which led the six-party talks to produce another key agreement on Feb. 13, 2007.

The agreement -- the "initial" action to implement the 2005 joint statement -- was to carry out the first part of a broad denuclearization process comprised of the shutdown, the disablement, the declaration of all nuclear programs and the dismantlement.

Despite the agreement to conduct the shutdown and allow IAEA inspectors within the next 60 days, Pyongyang did not implement it in earnest until after Washington lifted the BDA sanction in late June.

On Oct. 3, the six-party talks produced another agreement as the "second-phase" action to enforce the 2005 joint statement. It was about the disablement and the "complete and correct" declaration of all nuclear programs.

As a show of its disablement gesture, the North detonated a cooling tower attached to its 5-megawatt reactor in June 2008. In October, Washington took Pyongyang off the list of terror sponsor states.

But the last round of the multilateral talks in December 2008 broke up amid an intense conflict over the process of verifying the North's denuclearization steps.

Strategic patience and maximum pressure

When the Barack Obama administration took office in 2009, hopes emerged that the U.S. might take a softer stance on the North. However, Washington, along with the conservative government in Seoul, maintained a tough stance, vowing not to reward "bad behavior."

Against this backdrop, Pyongyang fired off a long-range ballistic missile in April, which triggered a U.N. Security Council condemnation. It then said it would not join the six-party talks and that it would not be bound by any previous agreements from the talks.

In May, the North conducted its second nuclear test, calling it a success.

The Obama government, however, renewed engagement with the North, which resulted in the so-called Leap Day deal in 2012. Under the deal, the North agreed to put a moratorium on nuclear and ballistic missile tests in exchange for "nutritional" assistance.

But the North fired off a long-range rocket in December 2012 under the disguise of a "peaceful satellite launch." Washington took the provocation as a breach of the Feb. 29 deal and later turned to a policy of what commentators called "strategic patience."

The strategic patience was criticized as a "strategic coma," as Pyongyang's nuclear program, along with its ballistic missile capabilities, continued to grow.

The North conducted the third nuke test in February 2013, the fourth in January 2016, the fifth in September 2016 and the sixth in September last year.

After the Donald Trump administration was launched in 2017, it got tougher on the North with its "maximum pressure" campaign, featuring tough economic sanctions, diplomatic isolation and threats of military action.

Trump's pressure policy, coupled with Seoul's mediation efforts, has led Pyongyang to come out for the ongoing dialogue.

Source: Yonhap News Agency