How did we get here?
North Korea has conducted five nuclear weapons tests, including two last year, and dozens of ballistic missile launches. Its nuclear weapons tests, which began in 2006, have become stronger. South Korea's Defence Ministry estimated that Pyongyang's latest nuclear test had a yield of 10 kilotons. In comparison, the "Little Boy" bomb dropped by the United States on Hiroshima, Japan, held 15 kilotons of energy.
Economic sanctions as well as six-party talks - which involved China, Japan, North Korea, Russia, South Korea and the US, and stalled in 2009 - have so far failed to stop the North Koreans from developing nuclear weapons.
How much of a threat does North Korea pose?
North Korea is thought to have as many as 20 nuclear warheads and missiles that can reach South Korea and Japan.
It is unclear if it has mastered how to miniaturise a nuclear device so that it can be delivered on a missile. In his 2017 New Year's address, North Korean leader Kim Jong Un claimed his nation was in the "last stage" of preparations to test-fire an intercontinental ballistic missile, the type necessary to reach as far as North America.
What does North Korea want?
In short, survival. As Professor John Delury of Yonsei University in Seoul told the BBC last September, North Korea's nuclear programme is about security above all else. "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," he said.
What is the stance of US President Donald Trump?
On the campaign trail, Mr Trump proposed to negotiate with Mr Kim over a burger. But he also likened Mr Kim to a "maniac", and said he would get China to "make that guy disappear in one form or another".
Yesterday, Mr Trump tweeted: "North Korea is looking for trouble. If China decides to help, that would be great. If not, we will solve the problem without them!"
What can China do? The escalation of North Korea's weapons programmes has strained ties with China, its only major ally. After the February missile test, China banned coal imports from its neighbour until the end of the year. Some argue that China could exert much more pressure since it supplies most of North Korea's food and energy. Mr Trump has said China could solve the problem of North Korea with "one phone call".
China's willingness to pressure Mr Kim is another question. It fears that a collapse of North Korea's regime might lead to a refugee crisis and - if South Korea absorbs its neighbour - create a well-armed US ally straddling its border.
After Mr Xi Jinping's visit to Florida, United States Secretary of State Rex Tillerson said the Chinese President had agreed to increase cooperation to control the North. Last Friday, the day that Mr Trump and Mr Xi held talks, China's Customs department issued an order telling trading firms to return their North Korean coal cargoes.
China and South Korea agreed on Monday to slap tougher sanctions on North Korea if it carries out nuclear or long-range missile tests, a senior official in Seoul said.
Would a pre-emptive military attack work?
It might succeed in taking out North Korea's known nuclear and missile sites, but the cost could be huge. The country has too many facilities spread out over too much terrain to destroy simultaneously. Even if North Korea reacted with only conventional weapons, its response, and South Korea's counter-attack, could be devastating.
Can Thaad shield against a nuclear attack?
The Terminal High Altitude Area Defence (Thaad) system being installed in South Korea joins a network of US missile defence systems across the Pacific. Whether they can take out a North Korean nuclear missile remains to be seen. Still, its advanced radars can help detect missile launches.
Is accepting North Korea's N-bombs an option?
Some analysts suggest that the best way to deal with North Korea is to accept it has nuclear weapons and seek its cooperation in preventing them from being proliferated. But others say this would lead South Korea, Japan and perhaps Taiwan to seek their own nuclear arms, undermining the Treaty on the Non-Proliferation of Nuclear Weapons.