North Korean leader Kim Jong Un attends a military parade in Pyongyang. (Jiji Press/AFP/Getty Images)

On Thursday, South Korea's national security adviser announced that President Trump is willing to meet North Korean leader Kim Jong Un by the end of May. The stunning declaration follows months of divisive back-and-forth rhetoric between the two leaders that, at one point, led Trump to proclaim Kim is a “madman with nuclear weapons.”

South Korean officials previously announced that North Korea is willing to hold talks with the United States on curbing its missile and nuclear weapons programs. North Korea also vowed to temporarily refrain from testing its missiles and nuclear weapons. The surprising declaration, if true, would be a major step toward potentially denuclearizing North Korea. Experts are skeptical, however, that the regime would be willing to bargain away its prized program. In August, North Korea's foreign minister told United Nations diplomats that the country's nuclear and ballistic weapons are not up for negotiation.

Here's what you need to know about the state of North Korea's nuclear program:

1. What did we learn about North Korea’s nuclear weapons in 2017?

North Korea conducted its sixth and most powerful nuclear test  Sept. 3, 2017, weeks after launching a new intercontinental missile that experts say is capable of reaching the United States.

While it’s difficult to know precise details, the explosion was five to 10 times as powerful as North Korea’s previous test a year earlier. Most experts estimated the weapon’s yield at between 100 and 200 kilotons, suggesting that the device was a thermonuclear, or hydrogen, bomb, as North Korea claimed. The test banished any doubts about North Korea’s ability to build a powerful nuclear bomb.

U.S. intelligence agencies estimate that North Korea has up to 60 nuclear warheads and that its nuclear program is more advanced than previously thought. In a confidential assessment, officials say North Korea has successfully produced a compact nuclear warhead capable of fitting in the payload of a ballistic missile.

2. What are the origins of its nuclear program?

In the 1960s, when North Korea was a close ally to the Soviet Union, Soviet leaders transferred nuclear technology and hardware to allow Pyongyang to develop a nuclear energy program. Those early nuclear reactors allowed North Korea to gain the technical sophistication, as well as a source of fissile material, for its future weapons program.

In the 1970s, North Korea obtained a Soviet-era Scud missile from Egypt and reverse-engineered it to make two of its earliest versions: the Hwasong-5 and Hwasong-6 missiles.

By the 1980s, North Korean scientists were beginning to fabricate uranium metal and experiment with detonating systems used in nuclear warheads. After the 1994 Agreed Framework pact was abandoned in 2002, North Korea began to race to build and test its first nuclear device, finally succeeding in 2006.

3. What are the things we don’t know about its nuclear program?

In addition to uncertainties about the program’s origins, we still don’t know whether North Korea has mastered all the technologies needed to put a miniaturized nuclear warhead on a missile and deliver it halfway around the world. There is, for example, no publicly known evidence showing that North Korea has surmounted the challenge of designing an intercontinental ballistic missile reentry vehicle that would enable a nuclear-tipped warhead to survive the trip through Earth’s upper atmosphere.

4. From which countries does North Korea receive its supplies?

Key assistance — including centrifuges for uranium enrichment, and possibly warhead design blueprints — was provided in the 1990s by Pakistani nuclear scientist A.Q. Khan. But over the decades, North Korea received help, some of it inadvertent, from dozens of countries around the world. North Korea developed a sophisticated procurement network that purchased needed technology and equipment, either through front companies or on the black market.

5. Why do nuclear weapons matter to North Korea?

From the perspective of Pyongyang’s leaders, nuclear weapons are the ultimate guarantor of the regime’s survival. A small, economically backward and isolated state, North Korea is surrounded by vastly more powerful rivals and adversaries, including South Korea, which maintains technical superiority in conventional arms. The country’s paranoid leaders also appear genuinely convinced that the United States will seek a strategy of regime change, as it did with Libya and Iraq. Nuclear weapons are regarded as Kim’s insurance card against any such attempt.

6. What are some restrictions on North Korea?

The U.N. Security Council has passed rounds of sanctions since then. In September, it imposed its toughest yet on the regime's oil imports and textile exports. The most recent sanctions put a further squeeze on the country's oil imports and placed more restrictions on its citizens working abroad.

Despite this, North Korea has been able to skirt the restrictions on its exports through ship-to-ship transfers and front companies set up in China.