North Korea fired missiles towards Japan, right? So where does the US come in?
Japan and the US are strong allies. There are over 50,000 US troops stationed in bases throughout Japan.
The missiles were fired by the Hwasong ballistic missile division that North Korea's state media said is "tasked to strike the bases of the US imperialist aggressor forces in Japan."
Three of the four missiles that were fired by North Korea as part of its military drills landed in waters that Japan claims as its exclusive economic zone.
Why did North Korea launch these missiles?
Pyongyang said it was in "response" to joint military drills South Korea and the United States hold every year. North Korea sees the military exercises as an invasion rehearsal.
South Korea is also a US ally, and there are 28,000 American troops there, as well.
In his annual New Year's Day address, North Korea's leader, Kim Jong-un, said his country would continue to develop its arsenal of missiles as long as the US remained hostile towards the country.
"We have reached the final stage in preparations to test-launch an intercontinental ballistic rocket," he said.
What's the problem with North Korea, South Korea and the US?
This complex relationship, or rather non-relationship, has a lot of history.
In short, South Korea and North Korea were once one country. Japan took over the Korean peninsula and made it a colony in 1910, ruling it for the next 35 years.
At the end of World War II, a weakened Japan surrendered to American forces in southern Korea and to Soviet troops in northern Korea. Elections to unify the two parts were to be held by US and Soviet administrators in the south and north.
Things didn't go according to plan, as the US called for a democracy and the Soviets for a communist state. There were separate elections and two leaders were chosen, effectively creating two separate nations in 1948.
Then there was the Korean War in 1950, which pitted the US, United Nations forces and South Korea against North Korea, the Soviet Union and China. Close to three million lost their lives until a ceasefire was agreed in 1953. No official peace treaty was signed.
The US maintains a presence in South Korea "as a strong deterrent against provocations from North Korea."
Where does China fit into all of this?
There's quite a bit of history between China and the Koreas as well, but that's not what Beijing's now worried about.
Earlier this week, the US started to deploy the first elements of its advanced Terminal High Altitude Area Defense (THAAD) anti-missile system to South Korea following North Korea's test of four ballistic missiles.
THAAD is designed to intercept and destroy short- and medium-range ballistic missiles.
China objects to the THAAD deployment, saying its territory is the target of the system's far-reaching radar. South Korea and the US have said the missile system is aimed only at curbing North Korean provocations.
Beijing, Pyongyang's biggest ally and trading partner, also hinted it would be willing to play the role of mediator.
"The question is: are the two sides really ready for a head-on collision? Our priority now is to flash the red light and apply the brakes on both trains," China's Foreign Minister Wang Yi said.
It called on North Korea to suspend its nuclear and missile activities in exchange for the US and South Korea halting the military exercises.
The US has rejected the proposal.
China's demand to halt the deployment of THAAD to South Korea has also been rejected.
Where does that leave all the parties?
The US is considering "all options" in dealing with North Korea. US Secretary of State Rex Tillerson is heading to Japan, South Korea and China for talks next week.
North Korea has warned of an "actual war."
South Korea says it will complain to the World Trade Organization about China's action and termed North Korea's actions "unacceptable."
Japan says "enough is enough."