After surrender of japan in WW II, USA and USSR agreed with divide korea south and north to unarm japanese army.

Because of Korea was the conony of Japan and korean were not helpful to fight againt japan, Korea had no power to get a independent from japan or USA.

Unfortunately, there was korean army for independent from japan and they were about to attack into the japan's territory.

Chinese army and Korean army for indepedent helped each other, they fought tother againt japan. 

But it was too late, USA air force attacked japan with two nuclear bomb and they got a victory before korean army began to attack japan.

At first, U.S.S.R build their own government in North Korea. The government's name was "Soviet Civil Administration in Korea" , but it was not for Civil.

It was exactly Military Government of USSR. USSR didn't want to lose their influences to North Korea. 

USA build Military Government in South Korea.

So USSR ruled the North Korea, USA ruled the South Korea.

In this environment, Kim Il-seong who supported USSR build Socialist Country in North Korea and Lee Seung-man who supported USA build Republic Country in South Korea. I think two person has their own ambition, so they didn't want to unite country together.

Kim Il-seong in North Korea wanted to be a king and Lee Seung-Man in South Korea wanted to be a president for a long time by build a new contitution.

There were so many people to stop their trying to split korea, but they were killed by Kim and Lee. Their effort to the united korea and preventing war failed.

In 1950, Kim Il-seong attacked South Korea to build a united socialist country. He thought USA didn't help South Korea because USA announced "Acheson line". 

The Acheson line was the defending line that defend japan when socilist country like USSR or China attack the democratic country side.

In fact, North Korea's army was very strong. First, the army was well trained to fight againt japan in WW2 and has many experience. The army helped socialist army of china and made chiang kai-shek defeat. The socialist army of china said that we couldn't win the victory without their help.

Second, USSR supported North Korea and they gave so many weapons to North Korea.

As far as you know, south korean and U.N army fight off the north korea. But the war was not finished. North and South Korea is in a truce.

Despite of south korea's denial China, North Korea and US army agreed with a truce, but South Korea didn't agreed. South Korea thought that the war can occur whenever they want, so president Lee wanted to end the war with support of US army.

This is why korea divided into south and north.

First of all, japan ruled korea as their colony so korean had no chance to independent. And many of people who fight for the independent divided socialist and publician. Second, Kim in North korea and Lee in South Korea were guilty in my country's history. Kim killed so many people to optain his own power. Lee caused this situation to get his own politic power. After war occured, president Lee recorded his voice for the radio that I'm here and don't worry. I'll fight with you before I ran away from his place. And he destroyed han river bridge to run away from North korea's army safely.