North Korea tested a hydrogen bomb on Sunday, raising fears that Pyongyang is getting close to constructing a nuclear-tipped missile that could reach the US.
The weapon was the most powerful North Korea has tested to date, with separate estimates putting the explosive yield at 50 or 120 kilotons.
To put that in context, the nuclear bomb dropped on Hiroshima in 1945 -- which instantly killed 80,000 people -- created a yield of 15 kilotons.
Here's what you need to know to get up to speed.
Why is North Korea testing a nuclear bomb?
A nuclear weapon is the ultimate survival mechanism for an isolated regime with little influence and few friends.
Many experts believe North Korea would not use its weapons first. Kim values the survival of his family dynasty and the regime. He knows the use of a nuclear weapon would start a war the country could not win.
Kim also craves international recognition -- and a nuclear arsenal is one guaranteed way to make the global community sit up and take notice.
"North Korean leaders know that dead people do not need money, and they believe that without nuclear weapons they will be as good as dead," Andrei Lankov, a professor at Kookmin University in Seoul, South Korea.
Are we going to war?
US Defense Minister James Mattis warned of a "massive military response" to any threat from North Korea against the United States. When US President Donald Trump left church on Sunday morning, he was asked if he'd attack North Korea. His answer? "We'll see."
While the US possesses overwhelming firepower compared to North Korea, any American strike on North Korea would likely expose neighbors South Korea and Japan to devastating casualties, analysts say.
Plus, with two launches of long-range missiles this year and the latest hydrogen bomb test, the US homeland may now potentially be at risk of a nuclear strike.
Experts say it's very difficult to verify North Korea's claims, but the very possibility of such a scenario makes the risks of any military action unimaginably high.
What happens if Kim attacks?
If North Korea were to strike first -- and last month Pyongyang threatened to send four missiles to the waters off the US territory of Guam and later sent a missile over Japan -- the US has a number of defenses in place.
They include the anti-missile defense system THAAD, which shoots down short, medium and intermediate ballistic missiles, and the ship-based Aegis system, which can track 100 missiles simultaneously and intercept them.
These systems, which analysts liken to a bullet taking out another bullet, could in theory take down a missile with a nuclear payload without detonating it -- although the radiation emitted would still pose risks.
What other options does the US have?
It boils down to two: Sanctions and negotiations. The United Nations Security Council has been trying to stifle North Korea's nuclear weapons program for more than a decade -- with little success.
There's not much left for President Trump to go after, although it could yet target crude oil -- something North Korea exports for foreign currency and imports from China, and its textiles industry.
On the campaign trail, Trump floated the prospect of sitting down with Kim, potentially over a burger, but, in office, he has talked tough on North Korea. On Wednesday, he appeared to rule out negotiations, saying on Twitter that "talking is not the answer."
The US position, however, has long been that it is willing to talk with North Korea -- but only on the condition that it abandons its nuclear missile program.
Some analysts say the US should accept a freeze -- allowing Pyongyang to keep its nuclear missiles but refrain from testing and developing any more -- to get North Korea to the negotiating table.
"It is a choice between bad and worse, and in this case all alternatives to the freeze are indeed, worse," said Lankov.
Who else could help?
Trump has prodded China to rein its unruly neighbor but many analysts don't share the White House's confidence in Beijing's ability to force change from Pyongyang.
While China has consistently supported UN sanctions on North Korea over the country's nuclear and ballistic missile testing, the US complains Beijing does not do enough economically to pressure Pyongyang and threatened targeted sanctions on Chinese companies.
"Despite Washington's hopes, China won't solve the North Korea problem, regardless of how often the Trump administration insists that it can or must," Jennifer Lind, associate professor of government at Dartmouth College, wrote for CNN last month.
"China worries most about political stability on the Korean peninsula. The Chinese fear that serious economic pressure would risk causing Kim Jong Un's regime to collapse, which could unleash chaos on the peninsula, and usher in a variety of long-term problems."
-Copied-