Huns, Vandals and Visigoth :)
The North Koreans almost defeat South Korea, by forcing them onto the Pusan Perimeter.
They are important because they just are
I am not going to answer in a paragraph of 125 words, but I will give you the answer to the first question; The United States didn't want another country to fall to communism. They officially got involved after the attack of the Gulf of Tonkin (U.S destroyers were stationed there and a radio came in that said they had been fired upon by North Vietnamese forces). (45 words BTW).
It brought the USA into a war that up till now they were trying to stay out of. It also caused the creation and use of the first nuclear bomb