Civil defence (or civil defense) is a system used to protect the civilians of a state from coming under attack. It involves plans for preparing for and responding to emergency situations, plans for evacuation, and plans for how to recover from attacks. In some countries, civil defence is seen as an important part of military defence. Programmes of this kind were first explored at least as early as the 1920s. They have been used in many countries, especially since the threat of nuclear weapons has become known.