Nuclear weapons are indiscriminate and even some modern so-called “small” or “tactical” weapons are more powerful than the bombs that killed 140,000 people at Hiroshima and 74,000 people at Nagasaki. They do not distinguish between combatants and non-combatants and their use would kill, injure and maim civilians in huge numbers. This means their use would almost certainly constitute a war crime under the existing laws of war.

A single nuclear weapon would likely kill hundreds of thousands of civilians and injure many more; radioactive fallout could contaminate large areas, including in the country that used the weapon, particularly if used against a nearby target which would be the case in the Middle East.  There are online resources available to predict these impacts, such as Nukemap.

It takes around 10 seconds for the fireball from a nuclear explosion to reach its maximum size. A nuclear explosion releases vast amounts of energy in the form of blast, heat and radiation. An enormous shockwave reaches speeds of many hundreds of kilometres an hour. The blast kills people close to ground zero, and causes lung injuries, ear damage and internal bleeding further away. People sustain injuries from collapsing buildings and flying debris. Thermal radiation is so intense that almost everything close to the detonation  is vaporised. The extreme heat causes severe burns and ignites fires over a large area, which coalesce into a giant firestorm. Even people in underground shelters face likely death due to a lack of oxygen and carbon monoxide poisoning.

Following the effects of the intense heat and the blast, there is the devastating impact of radiation poisoning. Nuclear weapons produce ionising radiation, which kills or sickens those exposed, contaminates the environment, and has long-term health consequences, including cancer and genetic damage which people can pass down to any children they may have later in life.

The use of nuclear weapons would contaminate large areas with radiation. Medical workers and first responders would be unable to work in these areas. Even a single nuclear detonation in a modern city would strain existing disaster relief resources to breaking point. Displaced populations from a nuclear war would produce a refugee crisis that is orders of magnitude larger than any we have ever experienced before. You can read more about the impacts of a nuclear detonation on health care systems here. 

Casualties from a major nuclear war between the US and Russia would reach hundreds of millions and according to recent research up to 5 billion would die from famine due to “nuclear winter” where sunlight would be blocked by soot and particles from the explosions. Even a “limited” regional war involving 100 nuclear weapons could have devastating global climatic consequences due to the effect of nuclear winter.