When Albert Einstein theorised that Matter and Energy are equivalent according to his equation E=MC² he was asked how he intended to release the energy of matter.  He replied that there was no reason to expect that it could ever be released.

The equation indicates that there is a huge amount of energy in matter.

Of course since that time, other clever people have found ways of releasing a bit of it.  During the Second World War the allies became aware that Nazi Germany was attempting to develop nuclear weapons.  The allies responded both by disrupting the Nazi attempt and by developing their own.  The Joint allied effort succeeded in both things and atomic weapons were developed. 

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After this, nuclear power was developed and used for electricity generation.  However, both in the minds of the general public, and in fact, the two uses of nuclear fission are connected.

Nuclear power stations can be used to produce fissile material for use in bombs.  This is one of the problems with different countries developing their nuclear industries.  The government of the country may claim it is only for peaceful use. But it might not be.

Unfortunately, a wide range of different types of reactor can be used.  Sometimes Thorium Fission is claimed to be impossible to use for making weapons grade material.  Unfortunately this is incorrect.  Thorium Fission and even some types of nuclear Fusion could be used to make weapons, simply because they are sources of neutrons and could be used to bombard elements that can be turned into weapons material.

If course it is unfair to blame the nuclear power industry for nuclear weapons and their spread.  The problem lies with Human Beings.  We are a species that makes war, and we are very good at making weapons.  It is just that the weapons that can be made with fissile material are so powerful.  There will always be some danger that some rogue state will build these terrible weapons.  If they exist, they can be used.