Since the ascension of Kim Jong Un as North Korea’s leader in 2011, the country has posed an ongoing diplomatic challenge for the United States. Pyongyang has ramped up work on its nuclear program and claims it is capable of launching a nuclear-armed missile that would reach the continental U.S.

The reclusive nation carried out its sixth and most powerful nuclear test early Sunday morning, which North Korean state television swiftly described as a “perfect success.”

The regime claims it detonated a hydrogen bomb, and although analysts have expressed skepticism about that, Japan’s Chief Cabinet Secretary, Yoshihide Suga, said Tokyo could not yet dismiss the possibility that it was indeed an H-bomb ― a device much more powerful than the atomic bombs dropped on Hiroshima and Nagasaki during World War II.

Pyongyang successfully conducted its first two intercontinental ballistic missile tests in July ― a feat U.S. President Donald Trump had called an impossibility shortly before his inauguration in January. The launches triggered international alarm and condemnation, and sparked a battle of escalating threats between the Trump administration and the North Korean regime.

Former President Barack Obama had warned Trump that a nuclear North Korea would likely be the most pressing issue faced by the new administration.

Despite assurances from Trump that a North Korea that can hit the U.S. with nuclear weapons “won’t happen,” some experts believe such a capability may be within Kim’s grasp. Here are the components of North Korea’s developing nuclear program, and the West’s efforts to stop it.

KCNA KCNA/Reuters An undated photo released by North Korea's Central News Agency gives a view of the test-fire of Pukguksong-2.

Nuclear Weapons

North Korea has been working to acquire a functional, deliverable nuclear weapon for decades. Those aspirations began during the rule of former Supreme Leader Kim Il Sung at the close of World War II and began to take shape under the reign of his son, Kim Jong Il, who first tested a nuclear weapon in 2006.

As New York Times reporter David Sanger explained in an interview with NPR’s Dave Davies in late March:

“Kim Il Sung remembers that General [Douglas] MacArthur, during the Korean War, wanted to use nuclear weapons against North Korea and China. He was stopped from doing this. But it made a very big impression on Mr. Kim, and he knew that North Korea, to survive and deter attack, needed to have this capability itself. And he’s the one, the grandfather of the current North Korean leader, who started down this path.”

The North was able to purchase much of its initial nuclear technology from one of the founders of Pakistan’s nuclear program and bought centrifuges to enrich uranium from Libya.

The North is in possession of up to 20 nuclear warheads, according to an annual report released this week by the Stockholm International Peace Research Institute.

KCNA KCNA/Reuters An undated photo released by the official news agency shows North Korea's annual central report meeting.

Nuclear Tests

North Korea has so far conducted six nuclear weapons tests, all at an underground test site in the country’s northeast called Punggye-ri. Those blasts, which began in 2006, have only gotten stronger over the past decade as the North hones its weapons program.

As The New York Times noted, the first such test, conducted by current leader’s father, Kim Jong Il, had a yield of less than 1 kiloton, or the equivalent of 1,000 tons of TNT. The second, conducted three years later, clocked in at 2.35 kilotons. The North’s most recent test was the strongest ever. It caused a shallow magnitude 6.3 earthquake, and was estimated to be at least “five to six times” stronger than the previous test last September.

The explosive power of Sunday’s purported H-bomb is adjustable from tens of kilotons to hundreds of kilotons, according to a North Korean report cited by The Times.

Comparatively, “the Little Boy” bomb dropped by the United States on Hiroshima held 15 kilotons of energy. The College of Arts and Letters at the Stevens Institute of Technology released a series of maps in 2015 that show just how damaging such weapons would be to cities around the world.

South Korean Defense Minister Han Min-koo said Wednesday that in light of the North’s recent ICBM launch, he believes there is “a high chance” Pyongyang will soon conduct a sixth nuclear test.

The Missiles

Alongside nuclear development, North Korea has undertaken an active and at times successful campaign to launch ballistic missiles, with an ultimate goal of crafting an ICBM capable of delivering a nuclear warhead to U.S. soil. ICBMs have a minimum range of about 3,400 miles.

Shortly after the first ICBM launch, North Korean state TV claimed the long-range missile was capable of reaching “anywhere in the world” and could carry “a large, heavy nuclear warhead.” This has not been verified.

The North has displayed two types of ICBMs at military parades since 2012, the BBC pointed out, but both remain untested.

The Musudan, another North Korean ballistic missile, is thought to be capable of reaching the U.S. territory of Guam. It was tested eight times in 2016, only partly succeeding once.

In May of 2016, U.S. and South Korean intelligence officials announced the North has the ability to attach a small nuclear warhead on top of missiles capable of reaching much of South Korea and Japan. Experts have varied greatly in the past couple of months about North Korea’s ability to strike the U.S.

Missile Tests

Kim has personally overseen the test launches of several ballistic missiles. While the tests are meant to fine-tune the North Korean arsenal, they also serve a political purpose.

Several of the recent tests have been timed to coincide with important strategic moments for the region.

The one in February coincided with Japanese Prime Minister Shinzo Abe’s meeting with Trump at Mar-a-Lago. Another one, in April, was timed ahead of Chinese President Xi Jinping’s visit to the U.S. Yet another launch in March was conducted in retaliation for joint U.S.-South Korean military drills. The July 3rd missile test ― Pyongyang’s 11th this year ― was launched on the eve of U.S. Independence Day, and days before the G-20 summit in Europe. Kim dubbed it “a gift for the American bastards.” Three weeks later, the July 28 test came a day after the North Korean holiday “Day of Victory,” which marks the 1953 ceasefire in the Korean War.

The North also launched three short-range missiles into the sea off its east coast on Aug. 25, intensifying the harsh rhetoric between the regime and Trump. And on Aug. 28, it fired an intermediate range missile over northern Japan.

Pyongyang is flexing muscles that experts say are getting bigger.

KCNA KCNA/Reuters North Korean leader Kim Jong Un during the test fire of Pukguksong-2.

What North Korea Wants

North Korea’s nuclear aspirations, experts say, hinge on Kim’s desire to retain control of the isolated nation.

“Above all else, North Korea’s nuclear program is about security,” John Delury, a professor at Yonsei University in Seoul, told the BBC last September. “It is, by their estimation, the only reliable guarantee of the country’s basic sovereignty, of the Communist regime’s control, and of the rule of Kim Jong-un.”

Sanger, the Times reporter, echoed that view on “Fresh Air”:

“If you consider your objectives to be to assure regime survival, to make sure that North Korea remains the Kim family’s personal fiefdom, then they’ve pursued a pretty rational strategy, one in which loyalty is above all, in which even members of the family who challenge the leadership end up getting executed. And under that structure, the North Koreans, for an unstable, irrational regime have played a pretty good game since 1953.”

International Response

Kim has continued weapons tests despite increasingly strict condemnations from the international community. In January, the United Nations imposed its “toughest ever” sanctions on the country in an attempt to stifle the program. It issued its latest round of sanctions restricting foreign trade and exports of certain goods on Aug. 5, after the second ICBM test.

But such actions have failed in the past.

Last September, former U.N. Secretary General Ban Ki-moon said the North’s actions in recent years had led to an unprecedented state of turmoil in the region.

“Never in the past have I ever seen such kind of heightened tensions on the Korean Peninsula,” Ban said.

In light of Sunday’s test, South Korea has called for the “strongest possible” response from the international community, including new sanctions to “completely isolate” Pyongyang.

Trump tweeted Sunday morning that the hermit kingdom’s “words and actions continue to be very hostile and dangerous to the United States.” He also continued his criticism of China’s role in the defusing the situation, tweeting Beijing is “trying to help but with little success.”

Kevin Lamarque/Reuters U.S. President Barack Obama held a joint news conference with South Korea's then-President Park Geun-hye at the White House in Washington Oct. 16, 2015.

Obama’s Response

Under Obama, the United States tread carefully and refrained from any direct action against Kim’s regime. As a member of the United Nations Security Council, the U.S. helped increase sanctions against North Korea following its September 2016 nuclear test.

“The United States is realistic about what this resolution will achieve. No resolution in New York will likely, tomorrow, persuade Pyongyang to cease its relentless pursuit of nuclear weapons,” former U.S. ambassador to the U.N. Samantha Power said at the time. She added that such sanctions impose “unprecedented costs on the [North Korean] regime for defying this council’s demands.”

The U.S. also reportedly was engaged in more covert operations. Sanger and his colleague William Broad reported last March that the Obama administration had been working to sabotage North Korea’s missile program for years with cyber operations. Such initiatives, the pair wrote, may have derailed components inside the missiles either before or shortly after they were launched.

While evidence is circumstantial, Sanger said that in the course of his reporting the Times noticed some missiles, including the intermediate-range Musudan, had a failure rate of 88 percent.

“This for a country that hired a lot of former Soviet scientists after the Berlin Wall fell and after the Soviet Union dissolved, brought them to Pyongyang, bought a lot of their technology and in their early days had a very high success rates with the North Korean missile program because they were basing it on experiences that the Soviets and later Russia had had. And suddenly, their failure rate soared.”

Carlos Barria/Reuters U.S. President Donald Trump welcomes Chinese President Xi Jinping at Mar-a-Lago in Palm Beach, Florida, on April 6, 2017.

Trump’s Conundrum

Despite condemnation from the U.N. and the West, Kim has shown no willingness to halt his country’s weapons program. Trump promised on the campaign trail to rein in the hermit nation, but faces few good options to do so.

Observers looked at the first meeting between Trump and Xi in early April for possible signs of the administration’s North Korea strategy.

China could play a significant role in motivating Pyongyang to reverse course. The country has continued to trade with North Korea, and Chinese companies provide up to 40 percent of the foreign currency the North uses to trade internationally.

The Trump administration had ramped up its rhetoric ahead of the meeting, with Trump even threatening direct intervention.

“If China is not going to solve North Korea, we will,” Trump told the Financial Times in April. “That is all that I am telling you.”

In a series of tweets in July, the president expressed his frustration at China’s apparently insufficient efforts to exert its influence over the North.

“Perhaps China will put a heavy move on North Korea and end this nonsense once and for all!” he wrote, later adding: “Trade between China and North Korea grew almost 40% in the first quarter. So much for China working with us ― but we had to give it a try!”

In a statement soon after, chief Pentagon spokesperson Dana White announced that the U.S. remained “prepared to defend ourselves and our allies and to use the full range of capabilities at our disposal against the growing threat from North Korea.”

“The United States seeks only the peaceful denuclearization of the Korean Peninsula. Our commitment to the defense of our allies, the Republic of Korea and Japan, in the face of these threats, remains ironclad,” she added.

Her statement marked a significant escalation of the U.S. response. Shortly after the North’s missile test in March, Tillerson had a simple reaction: “The policy of strategic patience is over.”

Uhhhh what does this mean pic.twitter.com/PzRpi6xjVt — Jessica Schulberg (@jessicaschulb) April 4, 2017

Pyongyang’s ICBM launches and claims that it has the ability to strike the continental U.S. sparked a heated war of words between Kim and Trump, with the threatening in August “fire and fury” for North Korea.

Direct action presents major difficulties as the South Korean capital, Seoul, lies just 35 miles from the North’s border and within easy striking distance of the country’s non-nuclear artillery. Were Trump to act, Kim could set his sights on a city of 12 million people that has long been a friend to the West.

It’s also unclear whether strikes could effectively target the North’s program, as infrastructure is spread across the country and in some cases lies underground.

For now, only time will tell how the Trump administration will handle a country set on pursuing a nuclear arsenal at any cost.

This article was originally published in April 2017, and was updated on Sept. 3 with new information.