Not during the Korean War...
To protect the U.S from North Korea
North Korea is not going to take over the world. It's a tiny country with mostly outdated technology. North Korea does, however, possess nuclear weapons, which could cause a great deal of damage if they were ever used. While the country occasionally threatens to use its nuclear weapons, it probably won't anytime soon.
North Korea is a Communist state which is on the verge of economic and social collapse. It supports terrorism around the world. Its leader, Kim Jong Il, seems mentally unstable and not in touch with reality. If North Korea has, or gets working nuclear weapons, the possibility that it might use them is too great to be permitted. Other nuclear armed nations have demonstrated a responsible attitude and reserve those weapons as an ultimate deterrent, never to be used except in the last extremity. In general, no one trusts North Korea to behave that way.
Perhaps you mean testing, they haven't used any.There is no certain way to determine if or for how long they might suspend testing.
They share a border. Being such a big country a nuclear bomb can be used against them without bothering their neighbor countries. This puts them in a critical position.
Nuclear weapons were used twice in combat against the Japanese Empire in 1945. In order to decisively end the Pacific War Hiroshima and Nagasaki became the only cities (in so far) to witness a nuclear holocaust. Many nuclear tests have been conducted since, with hydrogen and neutron types also detonated. The last suspected nuclear tests were by North Korea in February 2013.
No, world leaders in general are not encouraging the use of nuclear bombs (which have not been used as weapons since 1945). However, Kim Jong Un, the leader of North Korea, seems to be an unfortunate exception.
The UN tried to limit nuclear weapons in many ways. For example, the UN simply wanted all nations to disarm itself with all WMD (Weapons of Mass Destruction), this includes Nuclear Weapons. Also when certain countries like Iran and Korea starts a nuclear program, the UN tries to make sure that none of it is used to make Nuclear Weapons.
If the North Koreans would start a nuclear war the earth would be pretty much torn up and polluted. The North Koreans' primary weapons are the nuclear warheads, so they spend millions of research money on it. The results would be devestating if used on the U.S as its current state is no good. The U.S would crumble.
No nuclear weapons were used in the korean war
Fear that the North Vietnamese would obtain nuclear weapons themselves (from Russia or China) and use them on South Vietnam or even on America.
None. No nuclear weapons were used in Viet Nam.