North Korean missiles can also hit the American mainland
North Korea says the latest UN sanctions on it amount to a declaration of war.
The UN ban comes as a punishment for North Korea's missile tests last month.
But what if a war with nuclear-armed North Korea were to break out?
Two North Korean experts spoke to the BBC. They have based their experience and conjecture on what will happen in the first few days or weeks of such a war.
In their words: The first thing that can be seen is the death of people at an unimaginable level.
In the first week alone, 3 to 4 lakh people will die - including military and civilians.
And by the end of three weeks, there will be more than 20 lakh deaths.
One of the two experts is David Maxwell, who has served in the US military in Korea, and the other is Bruce Beckett, a former Pentagon analyst.
David Maxwell says, "What a North Korean commander wants to do in this case is to make full use of the power of their artillery - to cause as much death and destruction as possible in South Korea."
"In the first few hours, literally millions of artillery shells and rocket fire from North Korea will continue to fall on South Korea."
Many of them will fall on the capital Soleo.
The number of North Korean reserve forces is about 6 million. So they can be called the fourth largest military force in the world.
"Whenever their leader Kim Jong Un thinks he is under threat - he will order an attack.
Fighting with North Korea would result in massive casualties
"The order could come for a number of reasons. If US forces or their allies attack themselves first, or take a wrong step - it could happen," said David Maxwell.
What will be the reaction of the Americans in this situation?
Bruce Beckett says their strategy will be to use air power, and try to hold North Korean forces as close as possible - taking time to lift heavy military equipment and ground troops into a fleet of tanks, trucks, armored vehicles, artillery, etc.
"It will take three or four days for Marine Corps troops to leave Japan. It will take three weeks to bring the heavy tanks from Texas."
What will be the end of the war?
"They will be able to fight for two to three weeks with the North Korean military's stockpile of weapons, food, fuel, etc. The main point of their plan will be to do what they can within this time. Because then they will have nothing."
As their military units begin to crumble, so will the intensity of the fighting. Only then will Kim Jong Un realize that he no longer has the power to fight.
"And when they realize it, will they have any reason not to use nuclear weapons?"
"They would think that it could kill millions of Americans," said Bruce Beckett.
