Kim Jong Un is building a nuclear missile that can reach the U.S. Can he be stopped? Here's everything you need to know:

What's North Korea's goal?
The tyrannical Kim dynasty has been pursuing nuclear weapons since the end of the Korean War. The current leader's father, Kim Jong Il, made the nuclear program the centerpiece of his regime's identity, to rally the starving, isolated country around a nationalistic goal and to extort aid from worried Western nations. Over the past two decades, the Clinton, Bush, and Obama administrations all brokered pacts requiring Pyongyang to give up various nuclear programs in return for aid or sanctions relief. But North Korea cheated on every deal. First, it violated the 1994 agreement freezing its development of nuclear reactors. In 2003, Pyongyang announced it had nuclear weapons; after the resulting "six-party talks" — among the U.S., Russia, Japan, China, and North and South Korea — North Korea agreed to shut down its program in return for aid. But it reneged once again. When Kim Jong Un came to power after his father's death in 2011, the pace of tests and bomb building accelerated dramatically. The Trump administration last week ruled out another round of talks. "The policy of strategic patience has ended," Secretary of State Rex Tillerson said. "It is clear that a different approach is required."

What has Kim been doing?
Threatening his neighbors and the U.S. with nuclear destruction. Over the past 16 months, his regime has tested two nuclear bombs and more than 30 missiles. It claims the most recent bomb tests involved hydrogen devices, which are much more powerful than simple fission weapons. It's not clear that this claim is true, but the explosions were nearly twice as strong as the one that destroyed Hiroshima. Experts now estimate that North Korea has built up to 20 nuclear bombs. Just as worrisome is that Kim's regime has openly boasted it is preparing to attack other nations. It called one missile launch late last year a dry run for "preemptive strikes at ports and airfields" in South Korea. And this month, it launched four ballistic missiles simultaneously that fell into the sea near Japan, in a test of a "saturation attack" to overwhelm missile defenses. It said the unit that fired those was "tasked to strike the bases of the U.S. imperialist aggressor forces in Japan."

Would it launch a preemptive strike?
Only if Kim feels certain that North Korea is about to be attacked the way Iraq was in 2003. But Kim's primary goal is regime survival, not suicide, and he knows any nuclear attack would result in his annihilation. So the real value of his nuclear program is as a form of blackmail, to prevent an attack on Pyongyang. Right now, North Korea has neither a reliable intercontinental ballistic missile (ICBM) nor a nuclear device small enough to mount on one. But it appears to be working strenuously to achieve both. "North Korea is on the verge of a strategic breakout," says Robert Litwak of the Wilson Center, "that would enable its leadership to strike the United States with a nuclear-armed ICBM." Many analysts believe North Korea is up to its old tricks: acting belligerently to pressure us into negotiations. "What North Korea wants," says Philip Coyle of the Center for Arms Control, "is for us to stop threatening them and to talk with them."

What about missile defense?
A U.S. missile-defense system known as THAAD, due to be deployed in South Korea this year, has run into political complications. South Korean President Park Geun-hye, who clinched that deal, was removed from office this month for corruption, and her likely successor, Moon Jae-in, wants to delay deployment; he favors easing tensions with Kim. China, too, opposes THAAD; it says the array could be used against its own missiles, thus undermining the principle of mutually assured destruction. Angering China is a problem, because it's the only country with significant influence over North Korea. While it long resisted U.S. pleas to cut trade with Pyongyang, it was sufficiently alarmed by the recent missile tests to halt purchases of North Korean coal. If THAAD goes ahead, China could lift those sanctions, because Beijing sees "the U.S. response, not North Korean provocations, as the primary threat to its security," says political scientist Minxin Pei.

Can Kim's weapons be destroyed?
The U.S. and South Korea frequently war-game a strike on North Korea, and Tillerson said last week that if Kim keeps ratcheting up his threats, "military action" would be an option. But the risks are high. North Korea's full military capabilities can't simply be bombed away. Western intelligence doesn't know where most of the regime's weapons systems are concealed, and some are hidden underground or inside mountains. But even if a preemptive strike somehow took out all the nukes, it could not immediately destroy the thousands of artillery units. North Korea could still retaliate by firing tons of shells and chemical weapons into the center of Seoul, killing hundreds of thousands of people. "There is no South Korean leader," says South Korean analyst Suh Choo-suk, "who thinks the first strike by the U.S. is okay."

Kim's murderous paranoia
Kim Jong Un has proved to be as much a threat to members of his own regime as he is to neighboring countries. In five years in power, he's ordered the execution of more than 100 top politicians and military officers, including his powerful uncle, Jang Song Thaek. Jang was executed by Kim's preferred method: A powerful anti-aircraft gun blasted his body into bits, and then flamethrowers incinerated the scattered remains. Paranoid and unpredictable, Kim is quick to turn on people in his inner circle. He has already replaced his defense minister five times, after having several of them killed. Last month at a Malaysian airport, his half-brother, Kim Jong Nam, was assassinated in a chemical attack that undoubtedly was ordered by Kim. North Korea's highest-ranking defector, Thae Yong Ho, warns that Kim feels his grip on power is slipping, and he is more dangerous for that reason. "Kim Jong Un's capability to wreak harm, not only to America, but also to South Korea and the world, should not be under­estimated," Thae said.