Newsround's guide to North Korea
For decades, the Democratic People's Republic of Korea - more commonly known as North Korea - has been one of the world's most secretive countries.
Its government doesn't like people from outside the country going there and finding out what's going on.
So what do we know about it?
Newsround takes a closer look at the East Asian country.
When was North Korea created?
North Korea was formally created on 9 September 1948, following the end of the Second World War, along with another country called South Korea.
The political differences between the two rival states led to the outbreak of a horrific war in 1950, which lasted three years.
Since then, North Korea and South Korea have been enemies and don't get on to this day.
Who's in charge?
Since 1948, North Korea has been ruled by three men from the same family.
Kim Il-sung was the country's first supreme leader, who was in charge until his death in 1994.
Control passed next to his son Kim Jong-il, who held power for 17 years.
In 2011, North Korea announced Kim Jong-il's son, Kim Jong-un, as its new leader.
Around a million people gathered in the capital city, Pyongyang, to hear the announcement.
What is life like there?
North Korea is home to more than 24 million people, who live under a form of communist rule, which strictly controls all areas of daily life.
People have to ask permission to travel around and it's difficult for visitors to enter the country too.
All TVs and radios are tuned to state channels and people caught listening to foreign broadcasts face harsh punishments.
These controls mean that most North Koreans may have little or no idea of world events, or how their country is thought of by the outside world.
Most North Koreans are extremely poor with things like fridges, washing machines, and even bicycles, hard to come by.
Many people rely on aid agencies, such as the United Nations, to provide food because there is not enough in the country to eat.
However, North Koreans who demand more from their government, demand a change in leadership, or those that just try to escape, are brutally punished and sometimes killed.
An Amnesty International report estimates that hundreds of thousands of people have been put in prison and labour camps, because they have disagreed with the government.
North Koreans seem to cheer and praise their leader at big public events but it's difficult to know the reasons for this because it's not possible to speak freely to people living there.
The North Korean Government says it's because Kim Jong-un is very popular with his people.
From an early age, North Koreans are taught that their leaders are like all-powerful gods.
But others argue that people could be cheering Kim because they are worried that they would get in serious trouble if they didn't.
Why are people worried about North Korea?
North Korea is trying to make nuclear missiles - powerful bombs that can cause devastating damage - which one day might be able to hit targets in the USA.
It's believed that North Korea isn't currently building any missiles that can travel to and strike the UK, which is around 8,400 kilometres away.
US President Donald Trump has been sending warships and submarines to South Korea, a country which America has supported for a long time, to try to get Kim Jong-un to stop his weapon-building plans.
But there's no sign that Kim is going to back down.
There have been many arguments between the USA and North Korea in the past and, although they've never been friends, things have always been worked out to stop a war in the past.