Effect of nuclear weapons on a country?

The use of a nuclear weapon on a country would be considered devestating, dependant on the payload and method of delivery of the nuclear device. It can destroy cities, and make the land infertile for many years afterwards please look towards Nagasaki and Hiroshima for more information.

The "effect" of nuclear weapons on a country would depend on what your perspective is: there is something called MAD (Mutually Assured Destruction) of which the cold war is an example, where the development of a nuclear arsenal by one country (USSR) was profilgate in the development of a nuclear arsenal in another country (USA) this happened in many countries.

Currently the body which is responsible for monitoring nuclear arsenals is called the IAEA (Internationl Atomic Energy Agency)