The possibility of countries engaging in a nuclear armed conflict looks unlikely but is still considered a very dangerous possibility. However, when it is used like the event that occurred in WW2 when the United States bombed the cities of Hiroshima and Nagasaki, it brought with the blast, enormous damage to its inhabitants.
Nuclear weapons are seen to be dangerous in three ways;
The initial blast: As far as the use if nuclear bombs go, only the United States have launched two nuclear bombs which fatally destroyed the cities of Hiroshima and Nagasaki.
Portions of the two cities were completely destroyed and it brought unimaginable death and suffering to its inhabitants.
These two bombs exploded with a combined destructive power of around 37,000 tonnes of high explosives. The level of devastation it caused is little when compared to the destructive power of modern nuclear arsenals.
The effects of Radiation: After the explosion of a nuclear bomb, within 20-30 minutes of the blast, radioactive ash from the mushroom cloud would drop, which will cover the blast site and the surrounding area, killing anybody who happens to be outside.
Also, soil and water bodies will be contaminated and the exposure of its radiation will cause birth defects for years to come. Analysts have made a model of a potential conflict between Indian and Pakistan, both of which are rivals in the Indian subcontinent. It is said that both countries in a potential nuclear conflict would kill between 50-125 million people which will be considered as one of the deadliest event in the history of the human race.
The environmental effects from the blast: the vast plumes of radioactive ash and soot created from the blast from the burning cities will disperse into the atmosphere, spreading around the area and blocking sun rays. The dimming which came as a result of the blast will cause a temperature drop of about 5 degrees centigrade. This will in turn affect the fertility of the soil and will make global harvest to fail for at least a decade, leading to millions of deaths. The depletion of the ozone layer will significantly destroy plants, aquatic and animal life across the earth. The radiation from the nuclear bomb will also cause irreversible damage to human and animals which will result in skin cancer, eye damage, and DNA mutation. Also this will in the future cause civil strife as a result of mass starvation, violent competition over shrinking resources and weather patterns will quickly shift.
Though when countries have diplomatic spat with each other like in the situation of North Korea and the United States, the use of nuclear weapons is not considered but though it is seen as a means of last resort. Countries that possess nuclear weapons show restraint but however, the possibility of using it still remains.
So far, the denuclearization programme initiated by the UN has not been successful as countries who possess nuclear arsenal are investing more resources in improving its destructive capabilities and are increasing their stockpiles.
Below is a graphic of countries that own nuclear weapons